Well you will get better clearance with the 10 rim with 4.5 backspace then the 9" with 4.5 backspacing because the tire itself will be wider on the smaller wheel. The tire will ride 1/2" further past the wheel on each side on the 1" narrow wheel. But the 1/2" will not matter. The same tire will give the same clearance for a 10" wheel with 5" bs as a 9" wheel with 4.5" bs.
Other then that either will work
__________________ 2010 Dark Charcoal Pearl Jeep Wrangler Sport S Automatic
4.88 Yukon w/Trac-Lok rear and Eaton ELocker Front - Synergy Gussets - B&M 70264 Trans Cooler
BDS 3" lift - BDS Fox 2.0 Racing Shocks
XHD front Bumper - Trektop NX - Goodyear MT/R w/Kevlar 35X12.5X15 on MB Chaos 5 Wheels - Thrush Turbo Muffler