I 50'd my Marauder last week and have always concentrated on strength, endurance & power stat improvements.
I've no idea if this is the best or accepted route to go down but it seems to be working for me.
I don't bother using specific named mods, I just write down the stats for each mod in each item I want to upgrade, and buy whatever will give me the best upgrade on the stats that I want.
Possibly not the best way to do it, but it works for me.