Aside from price, the only reason I can see to go with LP tanks over HP tanks is if you plan on overfilling an LP120.
For nearly every other LP tank there is an HP tank made with nearly the exact same weight and dimensions. The HP tank is just rated to a higher pressure and will hold more air at that higher pressure. So if your local shop won't fill to 3500 you can always use it at lower pressure and you'll have just as much air on your back as you would with a similarly sized LP tank. If you have some old regs with low pressure yokes, just don't get your HP tank filled all the way.
Basically, an HP tank works just as good as a similarly sized LP tank at low pressure. It just has the option to hold more air.
Except that the LP will always hold more air than the HP equivalent.
For example... AN LP85 holds more than an HP100. An LP95 holds more air than an HP120 and an LP108 holds more air than an HP130 at the same pressure. Oh, and LP tanks are typically cheaper and for many people have better buoyancy characteristics.