Okay, here's what I think:

The difference between wearing a janeau and taking amrit is simple.

Wearing the janeau is supposed to "purify" you and give you a spiritual rebirth. Guru Nanak Dev Ji said however, that he had seen men that wore a janeau but they still lied, still cheated, still stole form others and bascially lived the lives of sinners. He said that the simple act of wearing a cotton thread wouldn't purify his soul. He wanted somehing stronger, something that would actually work. But this doesn't mean that the janeau was totally useless. If you were a devout Hindu and you had the nessesary faith and love for God then it would work because you had the most crucial part: FAITH.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/sikh-sikhi-sikhism/23329-thought-possible-contradictions-relating-5-ks.html

By the time Guru Nanak Dev Ji was born however, people had turned the janeau into a mindless ritual. They believed that it would purify thier souls and save them without any work on thier part. THIS was why Guru Nanak Dev Ji wouldn't put on the janeau. He didn't want something that was only symbolic of his faith, he wanted something stronger, something that would further help his soul.

Taking amrit is different because you do not take amrit and then suddenly your soul is saved. No. It does not work that way. You have to WORK to keep your amrit. Taking amrit helps you actually physically cut back on the sins you commit everyday.

For example: When you take amrit you are forbidden from smoking or drinking. These two things are sins because of thier addiction. When you smoke or drinking pretty soon you get addcited to it and then that's all you can think about. Amrit saves you from this because even if you are tempted you know you can not drink it otherwise you will break your amrit. This also goes for eating meat. Some people may argue that we need meat to live. Well, in some cases that was or is true. For example, when humans used to live in the wild we had to hunt other animals to live. We had no other choice. Or in places like Africa, sometimes poeple are so poor that they MUST eat animals because they are the most easily accessible thing. BUT in rich countries like Canada or the U.S or Britian, we can afford to get other foods and NOT eat meat. EATING MEAT JUST BECAUSE YOUR TONGUE LIKES THE TASTE IS A SIN! It is murder. Amrit stops you from this too because meat eating is also a kind of addiction and it HURTS living creatures CREATED by God. Another thing is hair cutting (WHY do you bother cutting your hair? Who the hell are you trying to please? The world? Cause guess what buddy, the world will always hate you. There is no need to cut your hair! It's a gift from God and it is a PART of you. If you can cut your hair then why not your finger? Cause it doesn't hurt?). By taking amrit you are also prevented from committing adultry. (Admit it, adultry is just WRONG)
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=23329

See all the things taking amrit prevents you from doing? All the potential mistakes that could ruin you life? Also when you take amrit you are slowly improving your self and instilling good virtues into your life. More example of this can be found at: SikhiToTheMAX - Enabling Gurmat Knowledge

Also when you take amrit you have to read 7 baania. (5 in the morning, 1 in the afternoon and 1 at night) And even if you don't understand them, they WILL have an effect on you. No matter how small. It's like butter in a container. You can take the butter out, but some of it will still be stuck to the container.

So basically, amrit actually physically helps you to live a better life. It's not just symbolic like the janeau.

Hope this helps

waheguru ji ka khalsa waheguru jee kee fateh