From a newb that asked the same kind of questions . . . Decide your priorities and go from there.
In my case, the priorities were:
Out-of-water weight
The most gas I could get for the least tank
Usability - single or SM, fits on the dive boats I use, etc (ruled out 8" dia tanks)
Behavior as the gas volume gets low
No extreme buoyancy characteristics
I picked the Worthington X7-100 because -
- weighs ~ the same as an AL 80
- most gas I could get in a tank that is ~ weight of AL 80
- good tank for single or SM, according to half of ScubaBoard (who I asked) :chuckle:
- doesn't change trim characteristics as it gets low
- no huge buoyancy swing
Ruled out PSTs due to some supportability issues I was told about - didn't want to buy from a firm having issues.
On thing I missed was coordinating gas fills with my lds. Instead of discussing my needs, I simply asked if he could fill a tank to 3500. He said yes. However, that is only when he pulls out his gasoline-powered compressor; the electric one he has set to 3000 and it stays there. So, unless it is the summer 'load' time, I end up with short fills.
All the above because I have limited storage and budget and choose to not have a dozen tanks around . . . even if the idea is really appealing.
Hope this helps.
So, effectively you have 85s (actually 87s) when you fill your 100s to 3000. That's still a good amount of gas.