What is your plan for the car? Weekend cruiser, daily driver, street/strip, strip only, occasional autoX, dedicated autox, or full on road course car? Makes a big difference. What bolt pattern?
Big selection in 15" rim sizes? Uh, no. Maybe if making the choice between BFG T/A's and Cooper Cobras is hard, because there isn't much else for 15" rims if you go wider than a 225 for a street legal tire. There are a handful of tires in a 225/60/15, but do anything much wider or narrower and its down to a couple.
Real tire selection options don't start until you get to 17" or 18" rims. Meaning that you can choose between at least half a dozen tires in various sizes with different tread patterns, speed ratings, tread wear, compounds, etc. If you're talking about a weekend cruiser or a street/strip or strip only car than having that level of selection doesn't matter that much. If you're talking about a frequent driver, or something you want handling performance from, or an autoX or road course car then it's very important.