I've done all of the combos except for tanker. Honestly, one of my favorites melee set combos. All three definitely are forced to have altered playstyle differences, Each one, really felt like it highlights the differences with the three ATs. I built all three very similar (as much as I could). You can definitely feel the squish factor as you go from Brute - Scrap - Stalk. Regrdless of which AT you go with, you will be the OOOOOOH AHHHHHHH WOWWWWW on your team. Now, the SUSTAINED damage difference between a brute and scrapper isn't that big, but the stalker version? Umm....that's a different story. Because of ass strike, electrical melee on stalkers is pretty wildly crazy ST damage too. All three make entire mobs disappear, but a stalker is going to be able to solo an AV before you bore to death (assuming you can take a hit and live). Honestly, this is one of the combos where you really just need to weigh what you value more. Al three rock.
side note: I used to farm with my brute. I wouldn't do that with my scrap/stalk.
sider note: Thunderstrike is awesome.