The similarities depend on what kind of Sufism you're talking about. Chisti Sufi beliefs are quite similar to Sikh beliefs. In fact, Baba Fareed Jee was of a Chisti background. In terms of the concept of heaven and hell, these do appear in Gurbani. Although the idea of what heaven and hell is may be different, words like "rasaatal", "narak", etc. appear in text by the Gurus and by Bhagat Kabeer Jee and Bhagat Fareed Jee. Also, for heaven, words like "behkunth" appear in Gurbani.
The main difference found between Sufis and Sikhs include:
1.) Clothing, Symbols, etc.
3.) Importance of Geneology
Gurbani has lines like "Cchodo vays bhaikh chararayee, dubidah eh phul nahin jeo" (against having religious cloting, symbols, etc), whereas Sufis stress religious clothing and symbols
Gurbani says "Nachan kuddan mun ka chao" (against dancing for religious purposes)
Finally, Sikhs do not have a concept of the importance of Geneology. (some sufi groups do)
In the latter part of the fourteen century, a great movement came into existence in India. It was a movement that later made the political achievements of Akbar possible. This political upheaval was preceded by a wave of religious revival, headed in the north of India by such immortal saints as Kabir and Nanak. National movements always seem to arise out of some such religious revival. Kabir was a Muslim, Nanak was a Hindu; but Nanak was claimed by the Muslims as their leader, being called by them Nanak Shah; and Kabir is claimed by the Hindus as one of their great teachers, his chief Gadi being in holy Benares. This was a movement that was intended to unite Hindu and Muslim; and the two great masters, Kabir and Nanak, typified in themselves this ideal of unity. About the same period there came, with liberalising forces, a movement that afterwards went by the name of the Sufi Movement. The religion of Sind is Sikhism and Sufism. The Hindus in Sind are chiefly Sikh, the followers of the teaching of Nanak. Guru Nanak himself visited the north of Sind. The Sikhs of Sind are chiefly Hindu Sikhs, and have very little in common with the Punjabi Singhs. Sikhism found a strong foothold in Sind, perhaps because of the Buddhist influence there; the Sikhism of Guru Nanak contains in itself the original spirit of Hinduism, minus all the accretions of latter-day Brahmanism. So Sikhism has given back to the Sindhi the spirit of th eold religion which he had lost to some extent owing to the causes mentioned above.
But the influence of Sufism in Sind both on the Hindus and Muslims has been tremendous. Many of the great original Islamic families in Sind accepted Sufism. Shah Latif, the greatest poet and mystic of Sind, was a Kureshi of the family of the Prophet, and a lineal descendant of the Mughal House of Herat near Afghanistan. Sachal, the next great poet and mystic of Sind, belonged to the House of Khalif Umar, whose very near descendant, Shahabuddin, came with the Arabs and became the ruler of Sehwan. These great families have been the real repositories of the best that is in Islam; they have kept intact its culture.
Sufism is the mysticism of Islam; and Ali, the lion of God and son-in-law of the Prophet, is said to have been the first initiator and organiser of the mystic school of the Sufis; but later on the Sufi Movement took on special colour as in Persia. The great Sufis of Persia, the immortal Rumi, Jami, Hafiz and many other resplendent mystic lights, have shed their effulgent and glorious spiritual rays on Inida; to this day they are the beloved teachers of Muslims as well as of Hindus. Sind has had a full share of this bread of life from the Persian Sufis. Afghanistan also claims to be the birth-place of one of the greatest of Sufis, Senai, whose influence even to-day is not insignificant.
When Sufism as such first came into India cannot be ascertained. Of course the spirit and teaching of Sufism are completely found in the Vedanta, and in the latter-day saints of India; but the comparatively fresher flowers from Persia added a charm, a beauty, a fragrance, that enriched the mystic treasure. The Sufis of Sind are peculiar in the sense that the garment of their mysticism is neither specially Islamic nor Persian, but it contains in its warp and woof the threads of both the Indo-Aryan Sanatana Dharma and the Arabic-Persian mystic culture. In fact there is hardly a country in the whole of Asia, including India, in which the mystic thought of two great civilisations, the Indian and the Arabic-Iranian, is seen in so beautiful a union as in Sind. There is a good deal of Sufism in the Punjab, and Punjab too has had some very great Sufis, such as Bulashah and Mian Bahu; but many of the Sufis of Punjab were in close touch with Sind, as till comparatively lately Multan was a part of Sind, whose boundaries extended even as far as Cashmere. The Punjab has even now many Sufis, but Sind being singularly free from religious orthodoxy has absorbed more of Sufism than Punjab where, on account of different political conditions, social and religious restrictions are more manifest than in Sind. In Sind ant the present moment, there are numerous Hindus and amongst them some of the best brains of Sind, old and new, who are Sufis by religion. In fact, throughout Sind, the Hindu Amils are at ached to the chief centres of the Sufis, and are the main supporters and advisers of the holders of the Gadi.
This Hindu-Muslim union is a marvellous phenomenon in Sind. This does not mean that there are no political dissensions in Sind between the Hindu and Muslim, and that religious bigotry is altogether absent in Hindus and Mahommedans. As a matter of fact there has been enough of it, and it still exists in many forms and is bound to exist in some form or another while the present political policy, that divides race from race, religion from religion, caste from caste, Hindu from Hindu, Muslim from Muslim, exists. Of course these conditions are not due only to the present political policy; it is in a good measure due to other, deeper, causes that exist in human nature; and also to the very fact of the variety of religion and sects. But in Sind, owing to its history and other causes, there is less of religious bigotry; and the experiment of the union of religions is to some degree successful and can be witnessed with the physical eye, not merely in the imagination. If one goes round to the various important centres of the Sufis, especially on the chief days of celebrations, he will be agreeably surprised to see the marriage of Islam with the older Religion. It is the fundmental basis of Sufism that the Truth is one. As the Koran says : "There is nothing new that I give unto you, what I give is as old as the ages." Thus while the Islam of the Arab is old as the hills, as they say, the religion of the Hindu is old as the snows of the Himalayas - even older. Sufism found a congenial soil in Sind, and seems to have spread into every nook and corner.