Per a Google search:
The TI-83 Plus has about a 6 MHz processor while the TI-84 Plus has about a 15 MHz processor. ... The TI-83 also has 160 KB of storage whereas the TI-84 Plus has 480 KB.
Sounds like 84 is the better calculator. It is more expensive than the 83, but looks like it is cheaper than the 83 plus.