Which calculator would you reccomend?

[lemontea]

Don't sell your Prime unless you really need the cash.

Keep the Prime, as it will get better and better with every new firmware version. (Or maybe as I hope, one day someone will jailbreak it and turn it into a super fast 48GX...).

The other thing is, you may not even be able to sell it for a good price (that is, not losing too much money), and you might want to buy it again in the future.

I have a Prime, sitting in a box somewhere.

By the way, if you have a smartphone (Android or iOS) there are CAS (Computer Algebra System) applications which can help you with calculus. Some are free, some are paid. This is a tangent (pun intended) to your calculator question, but also related (the reason is, you don't really need a super powerful calculator, when you get super powerful CAS software on your phone and computer nowadays).

For example, I only use my calculators (11C, 15C, 48G) to ASSIST me when I do hand calculations, or to add up the shipping and taxes to my online purchases (haha).

Otherwise, I would have my iPad running PocketCAS ($10 I think), with a bluetooth keyboard nearby. On the PC/Mac you can run XCAS (free) in command line mode, or install GeoGebra (also free) for a cleaner user interface.

The 15C has pretty lousy Matrix calculations. You would be pretty annoyed trying to do it on the 15C (that's why I never do it on the 15C). But it is my favorite calculator. My 11C is my oldest one. My 48G is the one I use most of the time though.