[flameout]

I have used a TI-89 Titanium for several years. I have no experience with HP calculators, and am writing this under the assumption that this HP has a CAS.

When I first took the SAT in Fall 2006, the TI89 was allowed -- the rule, specifically, is that it must not have a full alphanumeric keyboard. It has been allowed since (I don't have any knowledge from before that date).

I have taken five mathematics courses at Oregon State University (still a high school student), and my TI89 was never a problem.

Specifically, I would compare durability and the CAS's ability between the TI and the HP. I've heard (and read here) that the HPs are more durable, but I don't have experience. I also recall reading somewhere that HP's CAS is less capable than TI's.

I would also look at the ease of use for both calculators -- I have no real issues with my TI89, but the HP might have oddly-named functions, or a lack of continuity in their naming/interface. The TI89 also has a catalog (and a "math" menu, and...) that helps you find functions that you don't know how to use (if you don't have the manual) or identify its parameters. HP's may not have the same thing.

I hope this helps.