As I understand, you do not want to have a lot of maintenance and hence going for AIO.
I have an IM25 AIO now, great quality tank. I also had a 30gallon in stand sump system before this one. The in stand sump system was a lot more easier/simpler to maintain.
Light bleeds into the back chamber on an AIO and calcareous worms and stuff grow on everything, making maintenance a pain. In addition to that, the chamber are a pain to access while cleaning the tank.
In stand sumps are much easier to clean, you are more likely to do maintenance on them as it does not feel like a chore.
Edit: There are great in sump system options in the sizes you are looking at.
I have an IM25 AIO now, great quality tank. I also had a 30gallon in stand sump system before this one. The in stand sump system was a lot more easier/simpler to maintain.
Light bleeds into the back chamber on an AIO and calcareous worms and stuff grow on everything, making maintenance a pain. In addition to that, the chamber are a pain to access while cleaning the tank.
In stand sumps are much easier to clean, you are more likely to do maintenance on them as it does not feel like a chore.
Edit: There are great in sump system options in the sizes you are looking at.