32 bit Small ec2-Instance or 64 bit Micro ec2-Instance?

It's not necessarily performance, it's memory addressing. If you're going to use a lot of memory, you'll want the 64 bit option. If it's less than 4 gig, 32 bit will work fine, as long as you're running 32 bit programs.

Given the amount of memory you cite it won't matter. Processor tuning will be less of a worry than disk access, architecture of your application design, and network speed. Your processor performance difference would be like worrying about how much weight the cloth seatcovers in your car affect your car's fuel economy while ignoring your tire pressure, driving behavior and engine tuneups.

A micro instance (0,6GB RAM) will outperform small instance (1,7GB RAM) only for short periodic CPU bursts, as it can use 2 compute units instead of 1.

For a web application small instance will be much better. But it is 4,5 times more expensive.

32 or 64 bit will have no difference.

64 bit doesn't generally help performance, and can impair it, unless your applications need to address more than 2GB of RAM. Since that's unlikely on a 1.7GB instance, I wouldn't worry about going for 64 bit. Your Small instance will generally perform better than a Micro, because it has a lot more memory (although the Micro can burst its CPU to higher levels than a Small for brief high workloads).