I would think it makes more sense to pay a little more to get a decent suit (hell, not even tailored, just off the rack) you own even if it sits in the closet than deal with the hassle of a rented suit. But that just might be my preference.
I'm assuming it costs like a hundred bucks to rent a suit because I have no idea. And on the Men's Warehouse site it's $150 for a Prom Tux.
Though I suppose if you go through big weight fluctuations it might make more sense to just rent.