Well we never know, some people may own both models. On Omnimaga.org there were even members who only owned Casio calculators back then, yet they were still active. They could recommend HP or TI based on their experience with both models. However, it's certain that the TI fanboys would defend TI models simply because they own one, even if they never bothered to try the others. Same thing would happen on Casio forums about other brands, altough they are rather civil about it, and on HP communities you would almost get killed just by mentionning you own a TI. To be honest, I would stick with TI, though. I remember seeing a HP49G graphing on Youtube and it seemed much slower than a TI-81, even if the processor is faster. Plus, they are apparently complicated to program and there's no TI-BASIC equivalent as easy as TI-BASIC on them. THis is not to mention that no matter how good the calculator could be, barely anyone use them online and in school so you would not be able to get much help and if you start programming there's no HP forum besides the unmoderated HP48 google group, so good luck getting recognition on your new games on HP if you get one in the future.
As for Casio, they are relatively cheaper than TI (I got a 9750G Plus for $25 brand new at Staples) and still not too bad, but as mentionned, the coding sucks on it. On the Graph 85/FX-9860G models, it's almost as fast as a 83+, but on older models, Casio BASIC runs from 5x slower to 10x slower. AFX 1.0/2.0 gotta be the slowest models (altough they have a lot of RAM compared to the 9860). One model I like a lot is the color screen CFX 9850G, but unfortunately you can't manipulate the colors like you want in BASIC, just in pictures. Except from Summer 2007 to Summer 2008, the Casio community has been less active than the TI community, but they got a good userbase. Just fewer forums.
I personally own a TI-73, 80, 81, 82, 83, 83+ (2, including one that is broken), 83+SE (broken), TI-Nspire, TI-85, 86, 89 Titanium, 92, Casio FX 7000G, 7700GE, 7400G Plus, 9850G, 9860G, 9750G Plus and AFX 1.0.
The TI-83+SE is still my favourite model of all time, because the 82 series are easy to learn to program for and easy to use and the TI-83+SE is the fastest of all, even more than the TI-84+ and TI-84+SE (because of the CPU ressources taken by the clock feature). I wish they did a TI-86+ or a TI-86+ Silver Edition. 96 KB of RAM and between 164 and 1540 KB of archive, yet, much easier to use than the TI-89 that replaced the TI-86