Basically, there are hardware differences between the 49+ and 50G. Go with the 50G. IMHO it's much better.
The 89T has a larger support group and a lot more programs available for it. TI owns the education market, so that brings along a lot of pluses too.
The 50G is a hacker / programmer's machine and I wouldn't trade it for any other calculator. I have my doubts if we'll ever see another calculator made that is as powerful and programmable as the 50G owing to present market conditions.