Well, upgrades after the fact are kinda moot... if you're gonna do that, then the Commodore 64 has similar type upgrades, including some CPU accelerators.
As far as the video goes, the VIC-II has more flexible color usage for tile graphics, can do more than 4 sprites per scanline, raster interrupt, and has hardware scrolling capabilities. You could argue that the Commodore's palette is uglier and I wouldn't necessarily disagree there. That's not to say I don't like the 9918 though... it was very influential and the followup designs (MSX and Sega on the Master System and Game Gear) that were backward compatible with it addressed pretty much all it's shortcomings.
CPU was much faster on the 64.
More memory available in the whole 64K address space.
You could certainly have a much more capable C-64 for a lot less money with a single disk drive and an RS-232 interface than a TI-99 with a PEB, memory expansion, disk drive, and RS232.
Keyboard was better on the 64.
I think TI XB was better than Commodore BASIC in all forms though.
The TI had that sweet speech synthesizer.
Still, warts and all I appreciate the TI and find it fascinating. It's a quirky machine. It was my first "real" computer (the TS/1000 was the absolute first, but got returned to the store and a TI replaced it), and then the C-64 came next.