flex is overall articulation of suspension... both up AND down... and in my honest opinion, the term flex to me, refers more to up travel, because you are flexing, or stressing the springs, rather than them being in a relaxed state...
but yes, back to topic, if you are planninig on doing a lot of 4x4ing, then you should go with a selectable locker... a little here or there, auto locker... virtually none, l/s, virtually nothing but 4x4, then go spool...
provided of course you are thinking about replacing it... my l/s torsen is just fine for what i need it to do... 4:1 torque bias