It depends what size/type of tires you want to run and what axles you have.
If 15" wheels will clear your brakes, odds are you aren't running tires big enough to need 17" wheels since you have Jeep/half ton axles. If you are looking at 35" tires or smaller go 15" wheels since pretty much all manufacturers make tires for 15" wheels in those sizes, and the loss of sidewall with a 17" wheel is detrimental.
37" tires are kind of the transition point in my mind. There are only a few manufacturers that make 37" tires for a 15" wheel, and at that point you are getting to the limit of what half ton axles can handle any way. Stepping up to one tons will require bigger wheels, so at that point 17" wheels are a no brainer, and with a 37"+ tire, you still have a good amount of side wall. Bigger than 37" with 15"wheels, you are pretty much limited to buying tires from Interco and Pitbull (Maxxis makes a 38/13.00-15 Creepy Crawler).
Obviously 17" is the most popular size for offload tires currently, so it is a safe bet. Tires for 16" wheels have been pretty much phased out for the most part aside from stock or slightly bigger than stock sizes, and I think tires for 15" wheels seem to be heading that same direction slowly. 20" and more recently 18" wheels have been gaining popularity as well.
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