Be aware that the capacity of tank is best calculated by using the top dimensions only, not the one inch per gallon rule, so my 75 gallon tank can hold just as many fish as the 90 gallon tank, they both are 48 inches long by 18 inches wide, the 90 gallon is taller, that's all. If I remember correctly, the formula is one inch of fish per 12 square inches of space.
The math: 48 times 18 equals = 864 square inches; now divide that by 12 and you get 72 inches of fish, but realize smaller fish are easier to maintain in quantity than larger fish, and this formula is for smaller fish. Now you might be tempted to say, I might as well use the one inch per gallon on the 75 gallon which yields 75 fish, not much over 72; however recall the 90 gallon also has the same square inch area, so it can only hold 72 inches of fish. So taller tanks can only hold the same amount of fish as their smaller cousins if the square inch area is the same.
This went on for so long I had a senior's moment and forgot why I was talking about this issue. If your 120 and 150 have the same square inch total on the surface, then the 120 and 150 both can hold the identical number of fish. The taller tank is nicer for longer finned fish like angels, but if your fish aren't demanding of vertical height, you don't need it. More height will mean more money in lighting and filtration and more of a demanding water change.