Definitely go with the larger size. They really aren't that roomy.
Some other things to consider about Reptariums...using the zippers a couple of times every day will wear them a lot and they do tend to fail or jam. The mesh is fairly heavy so it does block some of the UV from ReptiSun 5.0 bulbs more than a hardware cloth cage top. Some feeders like superworms have been known to chew holes in them over time.
I like ReptiSuns for temp or outdoor basking cages rather than permanent homes.