One other factor that I haven't seen mentioned, is how this saw fits in with any other saws in your collection...
Will this be your ONLY saw? If so, take a look at the typical wood that you cut, and go for the bar that fits it - however you should also consider the "two bar plan" as a way of handling those embarrassing moments when you get your saw stuck... As a "two bar" plan, I'd get a 16-18" bar for your primary use bar, and a 20" (or possibly even longer w/ a skip chain) for use on the occasional bigger log, or as the "emergency unsticker" bar. (This is basically what I do w/ my 7900, 20" for normal cutting, and a 28" for the big stuff)
Do you have a bigger "felling saw" and plan to use this as your limbing saw? If so, go with the 16 or 18" bars - the shorter bar will cut better and be lighter, but will make you bend over more...
Do you have a smaller limbing saw, and plan to use this as your felling and blocking saw? In that case you'd probably want the 18" bar - possibly the 20", but I would look at getting skip chains if using the 20"
Gooserider