this might help:
in reality, lions are going to do very well in either size tank.
what might your deciding factor is live rock.
if you go to the salt forums, people HATE 18" wide tanks with a passion in most cases.
why?
because even though you think you have lots of width, when you start putting chunks of irregular rock in there, it becomes skinny real quick. especially with a decent sized peice.
hence, the favourtism towards a 24" wide tank. which is the 150g in this case.
so, not only do you have more width to work with for live rock, you have 25g more water to play around with too, which is a good thing..
if you go with the 150g that is.