well, generally speaking i'm a chronic 'under-liner'
but if we want to get into specifics, comparing the SLX to the GT90 is a little like comparing apples to oranges. the SLX is purposely built to be a heavier line. it's one of the reasons that makes it such a fantastic line for rolls and speys.
the GT90 can of course do r&s but really shines for aerial casting. in aerial casts we don't necessarily need the majority of the line's mass near the rod tip as with r&s.
my set-up for general trout is one spool with a GT90, basically for upstream and another with an SLX for across or down.
once again, personal preference but the simple rig above lets me fish any which way and cover all possibilities on the stream and change them in a matter of a few minutes
cheers,
marc
ps- regardless of what the rod is made of....