I use a Saunders 16x20 V-track and can attest to its qualities. Even margins, easy to adjust with spring hinges to keep the blades up while you change papers. Its only drawback (also an advantage) is the weight of the easel, which is at least 20 lbs. If you go no larger than 11x14 they make a "slim line V-track" which will have the advantages but be much lighter.

edit: I should add that the v-track is also very large and may not fit under smaller enlargers. I can measure tonight when I get home if anyone wants me to.