Personally, I do NOT see this as a simple question.
In fact, it is a pretty good one.
Factors to consider:
How hot is it where you are?
Do you pay your electric bill?
* If not, how much grief will you get for running an ac?
What is your PRIMARY goal for this: cooler running, quieter computer or just maintaining acceptable temps?
Is noise an issue for you? (ac and air cooled pcs are noisier than water cooling.)
Water cooling by itself does NOT compensate for high ambient temps; the improved overall cooling can help, but a very hot room will still result in a hot computer.
Air conditioning costs a LOT more to run than a water cooled computer though, especially in a very hot locale. There is the side benefit of cooling the computer user too, of course...
I don't have any personal experience with the Exos 2, but I do have an orginal Exos, and it really is a pretty decent unit. I can build a better one for less money fairly easily, but then I am pretty well into water cooling as a hobby in its own right.
For me the choice would be simple: water cool the computer.
But I would also build something custom unless I could find a used Exos (or similar) unit for sale.