I used to dig the 410 + 115 idea, for a long time. But I finally ran into a situation where it really didn't work for me.
Had a gig the week before last where we had to push some serious volume, and my SVT-115E started struggling pretty bad while the SVT-410HE was just cruising. Luckily, I had the 115 on top of the 410 so it was closer to ear level and I could hear it.
Before our second set, I unplugged the 115. I found with the 410 on its own, I was actually getting more volume, and it sounded better from the stage.
I rented an 810 for a fairly big outdoor gig tonight, so by next week I might be on the hunt for a fridge.
Anywho, my opinion says go with 2 x 410. I like 15's too, but I've come to prefer the sound of multiple 10s.