Both have there merits and depends on priorities and situation. I prefer a sump as it leaves me more flexibility for equipment and larger water volume. People with AIO often have amazing setups though...again i say personal preference.
It’s definitely personal preference. I have used both types of systems and both work great. AIO easier to work with. Sump tanks hold more water and gives a bigger opportunity for a refugium.
Really comes down to preference. A sump looks cleaner since you can hide all the gear, but comes at an additional cost. Sounds like you wont need much room for large skimmers/reactors etc since you are not planning on fish so an AIO can easily get the job done. Personally I like sumps just to hide dosing lines cords etc