While the 2GB Pi is used by many the 32Bit OS does limit the RAM considerably. While the 4GB Pi may show that most of that 4GB is usable in reality it is not.
A 32 bit OS it natively allows the CPU to address up to 4 gigabytes of RAM memory where a 64 bit OS allows the processor to natively address much more which allows the 8GB Pi to be used and the 4GB at its full potential.
The formula for those who may be interested
2^bits
2??=4294967296
Zach Davis - AB0E