I've been away and couldn't "weigh in" earlier. The comrades above have already outlined most of the T3 advantages, but here's my view and some additional data:
T3. Hands Down. Not even a nanosecond of doubt - nothing to compare in my opinion. The T3 is a handheld and the T5 is a nightmare.
The flash memory capability of the T3 far outshines that of the T5, which is basically an internal SD Card you can't control.
A HARD RESET will wipe out "memory" you have that you thought you'd have afterward. Then what's the point? It is a strange type of memory storage/access indeed - both on the T5 and the new Treo - and it slows down the unit!! The new "pseudo-flash" spooks me and I wouldn't have anything to do with it. You can get almost 10MB of flash memory, safe from a hard reset, on a T3 by using a 3rd party app called JackSprat/JackFlash.
Card Export overcomes the virtual USB drive problem on a T3 but I'd add the following caveat:
I don't care for the fact that it duplicates exactly the 9k ROM version of a USB driver into RAM, along with USB-enUS. One or both of these must be deleted manually on uninstall, and you must use CardExport ITSELF to uninstall (unhook) first, then delete normally. Still the "pusb" Saved Preference and the USB driver_enUS are left as remnants. Make a note of that - otherwise it's a good workaround for USB recognition on the desktop. I don't know why it requires 2 copies -- ROM and RAM - but it duplicates the Version 5.0 driver you already have.
The addition of an SD Card and either ZLauncher or TealAlias will launch countless apps from the Card. A new "still secret" app, almost ready for release, will give you 256MB Virtual RAM on the T3.
Combine the BETTER attainable flash on a T3 (with a 3rd party app), and the use of apps on the SD Card using one of the two afforementioned 3rd party launchers, the vibrating alert, the LED charging light, basic PIMS reliablility and the cradle . . . . plus METAL construction, some "heft," and other advantages listed in above posts and I think the choice is obvious.
T3 all the way.