Hi All:
Either the 15x6 or the 15X8 inch rims will work with a 10.5 inch wide tire.
Which wheel width is best? As always, depends upon the use.
If it is for general on the road, highway driving the 15X8 is probably the better choice - this is the width the tire shop folks will recommend.
If the tires will be used off-road a lot, aired-down to 15 PSI or lower than the 15X6 wheel is a better choice, IMHO. The narrower six inch rim holds the bead of the tire better aired-down than the wider eight inch wheel will, and will help keep sand, pebbles, etc. out of the bead seat area (ask me how I know this
Perhaps the best compromise would be a 15X7 inch wheel? Just be sure to get the correct back spacing; factory wheels have 3.5 inch backspacing, while the typical aftermarket wheels often have 4.0 inch, but 3.75 inch BS is also available. Having too much backspacing will cause clearance issues with steering components, and potentail tire rub on the fenders when the suspension moves.
Good luck with your rig!
Alan