I don't want to keep upgrading.
Famous last words...
One of my easy rules: Get the largest tank you can for the space you have to fill.
Give the size difference between the 32 and the 45 (looks like about 22 wide vs 26 wide); go with the 45. I feel the 45 is also a nicer looking tank. BUT it is not a fully 'system' so you will need to do more work to get things going...
So, we know your budget (easily manageable with such small tanks), how much effort (diy, general assembly, research, and so on) do you want to put into it? For example. the entire point of the AIOs are to set it and forget it; add some stock and you are good to go, that is the theory anyway...
What do you want out the aquarium? This simple question is often not answered enough at this stage. If you are only going to want low maintenance corals and some clown fish, then they system will be different than an SPS dominated reef...
Lastly, if you already have your heart set on some specific fish, best get that out on the table before you purchase. When I bought my first tank (a standard 65 gallon), I was sorely disappointed to fine out several of the fish I wanted would not work long term.