The 120 is a 4 version of the 180. I have both and neither are easy to reach the bottom of.
I recently got the Deep Blue frag tank that is 80g[4x2x16"].
Very nice and easy to work on...I also got 2 of the 4x2 60g tanks also.
I would not go deeper then the 120 myself aswith specific fish the vertical space is wasted..
In theory If the 150 is 4x2 I can keep as many fish in my 80 that is 4x2,,It is foot print that is more important then volume IMO