I'd go with the 20 pound turkey as its easier to source. At least I don't recall seeing a whole turkey under 12 pounds. 12-14 pounds tend to be hens, 18+ toms.
When you get into the large birds, cooking them evenly gets tricky. So much so that if you stuff them, you're more likely to have salmonella issues.
The last while, I've taken to a disassembled technique which works well with a large bird. It's a little more work in sectioning the bird but the results are quite good. I talk about it in this turkey thread. Read the Washington Post link there which will detail the prep.
This year, the video is on youtube, which is very helpful to watch.
This is easier to carve being all disassembled as well.
Half the point of roasting a large bird is the presentation and carving at the table. I don't think the meal will benefit from a reheated bird. If you cover the bird when you take it out of the oven and let it rest, It will stay warm for a quite a while so there's really no need for a crazed last minute rush to get the bird out and carve it. Remove, rest, wait as needed while you take care of everything else. Carve, cover, wait, uncover serve. If you desperately need more time between roasting the bird and carving. you can put it an insulated "ice chest" where it's own heat will serve to make a warm holding chest instead.
I'm more inclined to make other things the day ahead and reheat them. Dressing, mashed potatoes, breads all reheat better and more easily than carved turkey.