Bigger is always better, so if i were you, i'd go for the 40 gallon. It would work if you put it on its side, and cut off the top, but it might take a long time. And 16 inches of planting space doesnt sound too big :/. The best enclosure you can go for, is one that you make yourself. A giant day gecko needs alot of room (my enclosure is 3x3x5 feet), and building one is surprisingly cheap. So, if you wont build one,id go for the 40 gallon.