We've camped at Mustang Island SP twice and
Padre Island NS once (you can get potable water on your way into PINS, they have it there).
IMHO, there is no comparing the two...PINS is far and away the better choice! We stayed in the Malaquite area and were within walking distance of the water. The solitude was incredible, but then we didn't go during a major holiday weekend either. The stars at night and the sounds of the surf lulling us to sleep was an experience we truly enjoyed. The winds are gustier at PINS than at MISP, but this helps keep the mosquitoes at bay. As you've indicated, sand (and salt) will get into everything.
MISP is like the others here have said, it's little more than a parking lot...for the RV's. Tenters will probably see little difference other than a greater influx of people.
There are coyotes and snakes in both parks, so take the proper precautions... especially at night when storing food. Keep the coolers inside your vehicle.
PINS doesn't take reservations, so it could also be your best opportunity since MISP may already be booked solid for the upcoming busy weekend.