O.K, when you run a 32bit application on a 64bit OS, your application performance might be good , but you would be failing to make us of the 64bit operation of the OS. It will not be possible to run the 64 bit application on a 32bit os because the address structure of the hardware are all different. When you run the 64 bit application on the 64bit OS, you are making use of the hardware structure and the addressing to the fullest extent, in short it would be 1:1 ration. Similarly for the 32bit application on a 32 bit os. For more information refer any system archiecture book, or refer the perticular system or vendor manual for their design detail. When you talk about the different architecture, you would be talking from the bus to the memory management and etc.
Reminder. Max size of Datafile is concerned with the 32-bit/64-bit OS. You can't move around your instance across boxes if you get the situation of creating TEST/DEVT instances for Production.
Once, you 64-bit, you have to keep going with 64-bit. This does matter alot if your database grows more than 50//100GB. For smmaler size, it will not make much diffrence, as you can always export and import.