The TI-Nspire CAS with Touchpad was just recently released by Texas Instruments. It has the same functionability as the one with the clickpad, and to be honest, after using both, I do not see the great advantage of the touchpad. If you are going to buy a TI-Nspire CAS, get the one with the touchpad. It is cheaper (I can't explain why it is cheaper than the one with the clickpad. I wondered the same thing.), and because the keypad is removable, it can be upgraded if a new keypad is ever released. It also will have better support from Texas Instruments because it is newer.
Another comparable model is the TI-89 Titanium. Although this is an older calculator, it does almost the same things as the TI-Nspire CAS. The Nspire CAS was built off the CAS in the TI-89. All of the flaws of the TI-89 CAS are present on the TI-Nspire. The TI-89 has much better programs available. You can get programs that can do Riemann Sums, for example, that are not available for the TI-Nsprie. Although the technology is older, it still does the job just as good, and in many cases better, than the TI-Nspire CAS.
I would reccomend that you get the TI-Nspire CAS if you are not interested in programs to enhance the built in calculator functions, but if you want to use programs to make your calculator better, buy a TI-89 Titanium. A good website for calculator programs is www.ticalc.org
Good luck with AP Calculus!