Best
While both are owned by the same tribe, I would give the edge to Spa. The high limit room at Spa is brighter and seems cleaner and has better free eats. The casino at Spa is smaller and nicer. Also, you are in downtown Palm Springs while you take a break instead of the outskirts (nothing around Aqua, though pool area is nice). One thing going for Aqua, if you are staying in the hotel during the week, is the free day pass to their spa.
Dealers seem nice at both but there are a few with attitudes (like most places) and the floor people don't hassle you. Of course, HL is watched pretty closely but when they are busy you can blend in with the locals just be very friendly. I gave one floor a 2-4-1 coupon to a nice restaurant (I just happen to find it on the floor and thought I should turn it in to the boss) and never felt cooler in my life, the heat must have been broken.....