If you have the space, then a sump is almost always better, speaking as someone with a 50 gallon AIO.
AIO's look very sleek and nice, they CAN look great, but in practice as you install nice equipment that actually works rather than the low-profile (and generally low performance) equipment that comes with the AIO, it will start looking significantly less sleek than before. You'll also just have so much more room in the sump.
I'm not trying to bash AIO's. Actually the contrary. IMO they're at their best with nano aquariums where space is a premium and you want a non-traditional stand and can do frequent water changes to maintain water quality without having to wear your back and knee's out carting 5 gallon buckets around.