I have stayed in both places multiple times (though the new PH--not until June 16--haven't seen the new casino there) and I've used the spas & exercise rooms in both places. Typically, I really prefer Planet Hollywood over Luxor, but if having a gym is that important to your vacation, Luxor makes better financial sense. When I'm on vacation, I do like to exercise, but I don't like to pay that fee the Vegas hotels charge. So I only use the spa & gym when I'm getting a massage or there are discount passes available. The rest of the time, I'm usually walking ~7-10 miles a day (I've worn a pedometer in Vegas) and figure for a few days, I can skip the treadmill. On some trips (primarily business trips) I've also brought resistance bands in my suitcase so I can do some exercise in the room.
In the end, you have to decide how much the gym access means to you for 4 days. You say the location of Luxor doesn't bother you, whereas I really dislike it. The pool at Luxor is fine though somewhat boring. Aladdin's pools were boring as well (I've read a couple reviews they're doing construction on the Planet Hollywood pools now--no one here has posted any recent reviews of that though) I really prefer the rooms at PH--better beds, easier to get around, etc. But as I've mentioned, I'm heavily biased! Luxor did have good drink service the past 2 times I was gambling there, and that's important too
Only two things that money can't buy
And that's true love and homegrown tomatoes