Definitely a tough choice here ... should you go the more memory and better processor offered by the T5 or the added versatility of Wifi offered by the T|X?? Essentially, the two devices are exactly the same except for those areas ... they both look exactly the same (except for colour), have the same layout of buttons, ports etc. and even have the same version of Palm OS - Garnet.
So I guess what you have to really ask yourself, is what am I going to do with this snazzy little device! Will you be using the WiFi at all?? Just remember, that in-built wi-fi will always be faster than the SD card add-on ... and if memory is your main concern, I've read that the T|X can support up to a 2GB card (just do a search on this forum). Processor wise ... hmmm, yeah, it is a shame that the T|X is substantially slower than the T5, but with that comes added battery life! Again, ask yourself what you will be doing on this thing ...
As I recommended to migoblu, the best thing is to take an SD Card (full of programs/files that you think you will typically be using on your new Palm) to your local electronics retailer, and test it out on the handheld.
Another thing to be concerned about is that the T5 (from what I've read), has encountered more problems than the T|X (read opinions from CNET.com). Also, with the extra memory on the T5, apparently, it is not considered to be one by the device, but is split into two ... one part for program, another for general storage ... although both you can write to. The T5 has the added functionality of 'drive mode', which allows it to be treated as an external hard drive when plugged into the USB port of a computer - but you can do this with the T|X through third party software anways ...
Good luck!!
Drewster