The GT90 is the only line I’ve cast to the backing, once, whilst practicing, not fishing.The SLX is a purposefully designed spey line. It's overweight compared to AFFTA but that's for a very good reason - to get some mass in the D-loop to enable a good roll/spey cast. As such, the SLX is the one you want on the river - it's fantastic for this, but poor overhead (in my opinion). The GT90 is the one I'd want on a stillwater for distance etc.
Probably couldn’t do it again