The shorter bar will pull the chain faster (less weight, less friction). Maybe a few seconds difference in time I would guess (assuming that the saws can adequately pull a 20" chain). It's not so much about the bar length as it is about the strength of the engine, all else being equal. A 30cc saw would have a hard time with a 16" bar in hardwood. It would be pretty useless to try a 20" bar on that size saw.

Shop the motor, engine size and even throw in the brand. Match it to what you want to do. I don't like to say brand but if you want to cut a chords of firewood for home heating, I'd steer you away from Poulan, Craftsman and the like.

I don't think you will find any difference in the speed of cutting with different bar lengths. What will make a difference is the engine power (size) and the proper sharpening of the chain. When I am cutting anything of significant diameter, I rock the saw a little, so that the contact between the chain and the log is shorter than the full diameter of the log. This way you are cutting always a 4-inch piece. If you have all 14 inches of chain contacting the log, you just load up the cut with chips. The rocking keeps thing less crowded in there. Oh, and don't get a huge saw for generally light work; there is more danger of mishandling it. Take it from a guy with a scarred knee.

I've always heard 3 cc. of enginse displacement per inch of bar length is ideal.