Availability of accessories/peripherals: This one's probably a wash, effectively. They both had plenty of add-ons and accessories, most of which aren't especially useful to all but the most hardcore users. Disk systems, multi-cart expanders, printers, and essentials...they both have 'em, but none will be cheap anyway.
Modern enhancements - TI99/4a. They both have stuff like SD and/or flash storage solutions, but the TI people are always monkeying with new hardware; the F18 is practically ubiquitous now. It's a bit slower on the CoCo side.
Game Library - gonna go with the CoCo 3 on this one, but mainly for the CoCo 1/2 library, to be honest. Many CoCo 3 games are pretty good but forgettable, with only a few true standouts IMO. Ditto for the TI99/4a library--several excellent games, just not enough of them (again, IMO). The CoCo 1/2 library is very expansive and contains many hidden gems.
Community Support - TI99/4a. Hands down.
Other stuff - Well, the TI has better standard joysticks and is Atari/Commodore compatible with an adapter, but the CoCo 3 has a better keyboard and Deluxe joysticks for both free-floating analog and pseudo-digital control, but few--if any--third-party alternatives (Wico made an Atari adapter, and possibly a trakball, but they're pretty tough to find). The CoCo 3 has a much better BASIC, standard composite out, and RGB support. The TI has some of the sharpest RF I've ever seen (along with the Intellivision), if that's important. TI games generally look (and definitely sound) appreciably better than CoCo 1/2 games, but are usually outclassed by CoCo 3 titles in most areas.
So...I dunno, man. Flip a coin? You won't go wrong either way.