If you tend to have a cold lower half, I'd say go full length. Sometimes I have a heck of a time keeping that pad under my legs, even if I tuck it into the footbox of my TQ it just keeps popping out.
The bright side of all this is don't be afraid to try some different designs out. If there is a hang near you, that will give you the chance to look at some different set ups. If you buy something and you don't like it, you'll get nearly all your money back putting it up on the for sale forum. Quilts from the cottage vendors really keep their value.