Since they are both the same brand, you're not comparing different quality, support, building or insulation methods... it's just the floorplan. And that's going really be your preference.
The SX sits too low for me, I thought I could downsize the tub and be happy in one of "sub-compacts" but I always feel like I'm sitting on the floor, up against a wall. But I'm 6' - 240lbs. My wife is perfectly happy in one. The corner seats on the SX have a lot "shape" to them, I don't like that, but maybe you do. I like more open seating so I can just relax and not feel confined in the space. If I want to lounge, I just lay back and float, like many people end up doing in a lounge seat anyway lol... so I like the Rhythm without a lounge vs the Relay.
So, if you're buying a Hot Spot for me, please get the Rhythm.
But if you're buying one for yourself, then get the one you like better.
