it depends also what you can find out and about - lithodoras are hard to find, as are callophysus, jurense and tigs seem to be around only a few months of the year.
in a 6x3x2 - i reckon you've got loads of options - i'd probably go for a nice tig and a jurense, i'd probably throw in something else to - such as a pteradoras or megalodoras, giraffe cats are nice too.
What else would you be putting in the tank though is another question?
I used to keep a single Paulicia in a 6x3x2 and that was awesome - bare bottom tank - the catfish (was 37") and a shoal of 150 neons - looked brilliant.
Do you want to get a catfish thats already a decent size or do you want to grow one on?
carl