If it is for your children and they are still Junior or High school, it is tough choice and I will state a few probably obvious points.
1st: if they have used RPN and like it, consider two 48's, a lowly 48g for less than $50 and a better 48G+ or X.
The 128K Ram is nice in higher math and science courses, but only used for games at the Junior High level and the way calcs get treated by kids, well, if you get only a gx and break it, there just may not be a replacement at that time available.
If your kids haven't developed a deep appreciation for Hp's just yet, the 49 is selectable and more stylish. It has much more RAM and it has a CAS. It doesn't look or feel like the Hp's of old. However, the keyboard though rubber feels pretty good, it doesn't feel like an Hp either. Do not confuse an Hp39 with the Hp49, as it is algebraic only and its keyboard is not as good as a 49 rubber keyboard. I would also venture to say that the 49 might just be tougher than a 48 for general book bag use. 48's sometimes had LCD screen breakage and the 49 has an acrilic clear screen over the LCD.
If you can look at them side by side and keep in mind that the 48 has about 2000 functions and the 49 has maybe 10-20% more. You can load a CAS on the 48 if you need one (it is an earlier release of the 49's CAS actually, same author).
I own both and find that I use the 48 more, but I like where the enter key is, whereas the 49 put the enter key in bottom right corner. If you are just starting out ie your kid, this probably isn't a problem, but to an old dog, it's just one more hard trick to relearn.
As for your thoughts about a 15 or 42, well the hard reality is that a 48 is ten times more powerful for 1/2 the $$$. It is not a pocket calc, but that is the breaks. You can still buy Hp32s (I haven't really checked, I just haven't heard of them being pulled from stores).
Just my $0.02