Etienne,
Back in the early 1990s there was discussion of the then-new 48G/GX, and of differences between them and the 48S/SX, that went into much more detail than the Forum responses to your question.
Yesterday I found a long item, "HP48 FAQ - Last-modified: 12/12/1994," which I think may give you more information you can use:
http://www.guzu.net/hp48/docs/hp-faq.htm
I would further note that there was little or no Forum discussion of reference materials that might help you explore the 48. I'd go for the 48G myself, and I'd start with the "HP 48 G Series User's Guide" and the "HP 48G series Advanced User's Reference Manual." Those might be enough.
But several non-HP books have helped me explore the 48G: in particular, four from Grapevine Publications, starting with "An Easy Course in Using and Programming the HP 48G/GZ."
Purchased as hard copies, all those books will cost as much as a 48G; some of them are available online, however, and perhaps others on the Forum can provide links.
Finally, I would note that I have had a lot of fun exploring problems provided by Project Euler; someone on the Forum recently said that he had solved about 100 of them, which I find very impressive.