Well, 2 things, on the foot end a full length does tend to get less spread out as your body profile around your feet narrows, so it would ride up more and in that regard it would be harder to see out at the foot end with a full length. Pretty minor, though. With a shorter UQ, you can rig it so its less under your head so that you can peak out around it if its riding up high in your setup. Again, pretty minor considerations, but you might consider a tapered full-length UQ if you want. Maybe one that is less wide than others, whether full or short length.
"The rain can touch me, but can I touch the rain" - Gordon Lightfoot