For best "upgradability" you would want to just get a regulator meant for big tanks and get a paintball adapter if you wanted to use smaller paintball tanks. I have paintball regulators (from Aquatek) and use paintball tanks - my reasoning is that it's much much easier to get paintball tanks refilled for me (I drive by the place that does them a lot, so it's easy to just stop in, rather than making a special trip) than it would be to drive the hour+ all the way across town to get a larger tank refilled, and CO2 for a larger tank isn't cheaper. I pay $4 each to get my 24oz tanks refilled and to get a bigger tank filled is $3/lb plus the extra driving... so I feel like I'm actually saving money with my paintball tanks
Plus paintball tanks are cheap (less than $20) so I actually have a couple of spares - no worries about running out and not being able to get it refilled ASAP.
I can't honestly recommend any of the disposable systems - the little CO2 tanks just don't last long, even in small aquariums.