I always prefer 12s (or even 10s) for tops over a 15. When you're running a sub, you're just not asking the tops to produce those lower mid and low range frequencies, so there is no need for it. And for the gigs where I wouldn't bring a sub, I'm not that worried about hard hitting low end. I'd rather have nice punchy mids from the tops and whether it's in my head or not... I think the 12 is going to be better in that range.
I agree with
@ittigger that you can start with one sub. I ran K10s over a single 181 for many years as my primary rig. And I rent or borrowed other 181s when I needed to go bigger than that. Easy to scale up when it's needed.