You might want to consider LP108's instead. Mine are Worthington and so might be hard to find, but I assume others make 108's. Most fills stations aren't going to be able to overfill your LP120's much. I dive lp108's and most often get them filled to 3600 or more. That's 147cuft, or slightly higher.
Fill capabilities are of concern here. How much pressure can your dive shop provide? Also, assuming a fill station can provide 3600psi you only end up with 130cuft in an HP120. If it was an LP108 you'd have 17 more cuft of gas! Furthermore, non tech shops seem to not even notice they are "LP" tanks. You generally at least get an overfill at least to the 3200 mark, even in tropical vacation diver locations.
If your fill station can only provide the more "average" 3200 psi. You're at 128cuft. In an lp108 you'd be at 130cuft. No big difference but more is better, right?
I bought the 108's when I was diving single tank and my sac was FAR higher than it is now. Still, it's a great feeling knowing I've got way more gas than I'll need.
The 8" diameter of LP108's kind of sucks. You get used to it, though. I've never been on a boat where the tank size was more than a minor annoyance.
. I love them for SM, and that came as a surprise, because when I previously dove them as manifolded backmount doubles, I thought they made me foot heavy. In fact, I broke them down from the manifolded configuration, intending to sell them, or maybe use them as BM singles, since I was no longer using them for BM doubles diving. I dove them SM a couple of times with my Nomad, and found that they worked really well- great trim, good buoyancy characteristics with a 5 mm wetsuit and the Nomad. I have used them with a Halcyon Contour as well, and that worked just fine. So, now I probably wouldn't trade them, because they are getting harder to find.
