A lot depends on how much time you have to dedicate to it. Full watercolors are going to take a long time to do a comic in, and you might get sick of the obligation of color. I only ever did one comic in color, and by the time I made the choice to end it, I was totally sick of having to do it.
A fellow colleague of mine uses a combination of greyscale and screentone, and it looks really good on his comic. I generally tend to use screentone/greyscale on mine and I feel like it looks good. I also like it because it involves the audience; they have to kind of meet you halfway and imagine a little bit, it's not just all given to them, including exactly what color everything is.
Color can also be one of those "make it or break it" things, because everyone tends to have a different color style. I've seen coloring I thought was gorgeous and breathtaking, only to hear someone I know absolutely hate it because to them it appeared garish, or bland, or some other thing that just didn't satisfy them. Color is one of those things that I don't think any two people feel the exact same way about, so a color scheme can really end up polarizing people.
I say greyscale and screentone combination is probably the best bet for both efficiency and quickness, as well as reaching an overall wider audience, with less chance of burning you out with the work required or putting people off who just don't jive with your color scheme.