There is a really good chart on the size/weight/buoyancy of all the popular tanks out there. (I will link to it if I can remember where it is). ...in any case, all large capacity steel tanks are not created equal. Some are thick, heavy buggers that are several #s neg even when empty. Some are much better.
If you want smaller tanks, than its hard to beat LP85's. At some point I want a set of LP108s (don't remember which make), but there are some that have really good weight/buoyancy for their size.
I currently have a set of LP95s that are not too bad, and a set of PST LP104s that feel heavy enough to anchor the Enterprise.
I suspect you were kidding about the step up to a CCR, but in reality.. You will need a bail-out bottle at that point anyway.. So I don't see how you can go wrong buying a decent sized set of doubles.
BTW: I keep the 72s around for my kids to use at singles, and when I feel like playing around with sidemount in open-water. I will add that I have seen divers using double 72's in Blue Hole (aka Jug) due to their low profile in a cave that most only get into with sidemount.