If you want to do symbolic math (calculus, differential equations, linear algebra, etc), then get the 50G.
You can get much of the same CAS functionality on the 48GX, but you will need to buy 48GX memory expansion cards (which will probably cost as much as the calculator itself), plus a 48 serial cable, and install lots of third-party software. And even then, the CAS will not be as up-to-date or as well integrated as it is on the 50G; it will run much more slowly, and the display will be smaller and have less contrast.
I bought a 48GX in the late 1990s, which I upgraded with two expansion cards, ALG48, ERABLE, etc. It works, but it's slow and clumsy compared to the 50G. For number crunching, the 48GX is still great. But for symbolic math, the 48GX reminds me of Windows 3.1 on a 286, while the 50G is Windows XP on a Pentium 4.
The only things I really miss about the 48GX are the big central ENTER key and the quality of the documentation. The 50G doesn't feel quite as solid as the 48GX, but the quality is still more than adequate.
Edited: 13 Sept 2007, 3:11 p.m.