Well then, you see the issues. The all in one is nice but it doesn't allow much modification in the future and you do lose space. But it is all in one, and there is some benefit to that. Otherwise, you can run a tank without the sump (with lots of LR, skimmer and circulation), but you will have things hanging on the tank. So that would mean drilling it with built in overflows would be good, but then you need the sump/refugium/wetdry (however you choose to go) below it. You can hide lots of stuff in a sump, and be a bit more adaptable with a drilled tank, IMO. But a lot comes down to personal preference. All in all, either way, you aren't getting a lot of fish in that 40g anyway.