From Ubuntu's website, 32-bit and 64-bit differences. Basically what it comes down to is how much memory you will be able to access. With a 32-bit system with 'Physical Address Extension' or PAE, you can hit a maxium of 64 gigabytes of memory that you can access. With a 64-bit system, you will have can have an almost unlimted amount, but most hardware will limit this to one terabyte of memory. When using a 64-bit powered machine, the percived speed of the system is much faster than a 32-bit machine. So from a users standpoint, this is great benefit.
Stright from Ubuntu's website
Unless you have specific reasons to choose 32-bit, we recommend 64-bit.
Whichever you choose, have fun with it!