LP vs HP tanks...this debate again.
Speaking as someone who owns both (single HP120 and a set of PST HP100 doubles, single sets of LP85s and LP95s), they both have their pros and cons. LP tanks are bigger and heavier per capacity at rated pressure. I can handle my HP100 doubles with little drama, it's going to be a long time before I double up my LP95s.
The big thing, IMO, is how well the tanks trim out for you. A friend of mine who normally dives her HP100s in sidemount tried my LP85s and she really liked them, because they are long and thin, she can trim them out really well. For singles use I love my LP95s, they fit me so perfectly and the extra amount of gas I get really is nice.
Also, don't get more tank than you can handle on land. I hear about folks diving doubled LP104s and LP120s and cringe, my back would be screaming bloody murder. Also, depending on your regular diving and gas consumption rate, a smaller tank is perfectly acceptable. Many of my regular dive buddies dive LP72s and they come back with more pressure than I do, with an extra 20-30 cubes of gas
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The basics of the argument come down to:
If you can get regular overfills (3000 psi +) in your LP tanks, go with LP tanks.
If you can't get good HP fills, get similar volume LP tanks.
There's a lot more to it, but before getting hung up on gas capacity vs tank weight or other figures, go out and dive as many different types of tanks as possible and see what works for you. Pay attention to your trim and how easy it is for you to hold trim while hovering. Then see how long the tank actually lasts you. Also, larger capacity LP tanks are generally much larger than their similarly sized HP counterparts. It can make a difference in your ability to swim the tank, and the extra resistance could negate the larger gas capacity the tank may offer.
But I re-iterate my earlier-stated point: trim is king, see what tanks make you feel the best and go get yourself a pair (or more).
Peace,
Greg