I have 2 Lp 125's (2400 psi) and recently got the 149 (3445 psi) and I don't make any weight belt adjustments going back and forth. The shop started giving us trouble about filling the LP to over 3000 psi, so i bought two new tanks: a 149 and a 117.
My friend uses the 149's as doubles and with 2 80 cuft stage bottles, I really don't know how he swims. The single tanks work great for me and I can often get 2 or even 3 recreational dives on the single big tank, especially if I whip a little gas off another tank for the third dive. When diving with a 3 mm full suit, I wear zero lead and I am actually heavier than I would like.