you just run the risk of running out of air faster, especially since a spooler is a long way from "efficient".
Smaller tanks are nice if you can get 4500 fills consistently. It's odd though....people have started using shorter and shorter tanks, et still bag on short drop forwards....when they do the same thing.
Honestly, with an HP tank, your not going to be shooting fast enough to GET shootdown. Your reg needs about a 200psi difference in pressure, that would mean 300-psi of shootdown for most guns. I mean if your pushing the absolute performance edge it makes a difference, but a tank is VERY rarely the shootdown culprit.
It all comes down to personal preference. I use a 68 just because I have longer arms and it puts my marker right where I want it. If you intend on keeping the proto, I would definately keep the 68, as it was said before, spoolers are not that efficient (hella good markers otherwise though)