You can't really compare the two, Talk Boxing is an art all on it's own, originating as a Guitar effect, Peter Frampton although he didn't originate anything, he made this effect very popular with his live performances. Your probably more familiar with Roger Troutman though, who took this to an entire new level with funk. Depending on what kind of sound you want to achieve, if you want a legitimate "Roger Troutman" style talk box effect, your going to need to get a setup, and practice for several years, or pay someone to do it (Bosko). You can't really mimic that sound with anything else.
Auto-Tune actually uses a phase vocoder to correct pitch. You can use the Antares software, or get the rack mount, or both I suppose. This is nothing like a Talk Box and can't be compared to one, it's used really just to correct pitch, that's all. The T-Pain Auto-Tune effect is just an effect, nothing more. If you want that sound, there are plenty of tutorials on how to use Auto-Tune and achieve it.
A Vocoder just mixes signals, gives a robotic type effect, through a multiband filter, you can input your voice and achieve that, it's rather dull and not very pleasing. It's popular with electronic music and has been since the 1970's but it's origins go back to telecommunications, encryption, and so forth.
With a talk box your actually controlling the output of the musical device (guitar, keyboard) through a tube with your mouth. You control almost every aspect, other than the note your are holding it in, and can basically re-create whatever sound you want with it. It's an art all on it's own and can't really be compared to anything.
So, the question should be, what kind of sound do you want? Simple.