That's a tough call. I think the Polaris is probably the way to go, they've been making play-market ones for a long time. Me, I would let width control everything. Get the top of the line one you can get and still keep the 50" width. Much wider than that, and it seems like it would be a real hindrance.
Also, I would consider upgrading the seatbelts to something more like a harness. My buddy's son really tore himself apart when he came out of it and the roll cage crushed his leg. He wasn't wearing a seatbelt, so I'm sure that would've worked, too. But in a rollover situation, you're far better off in a harness.
08 700XX (Gray, L8ton Racing Stealth Bumper, 13 tooth, 21" Razr2 fronts, stock rears, Precision Pro stabilizer, Fox Evols and Podium Xs, Teixeira Tech stock-length upper A-arms, Teixeira Tech stem, L8ton Racing 9/16ths sway bar, ProTaper Contour bar, Shock & Vibe clamps, EHS lid & programmer, HID headlights)
08 Outlander 800
08 Yamaha Big Bear 400 4x4