Ever since I discovered that the Eviscerator benefits from the melee training perk, I find it hard not to use 20 weight constantly. The ability to bring an evicerator, move super-fast, kite bosses, block them AND carry another Tier 4 weapon is just too good to pass up just for medic grenades. The strength of survival with 20 weight is that you can dip into two different classes to combine their strengths, otherwise you just end up being a weaker version of another class.
Eviscerator + RPG + Medic Pistol I my usual loadout, though you can experiment with Seeker six, Rail Gun, AA12 and other high tier weapons.
Potentially with 15 weight, you could go...
1: Eviscerator + Double Barrel + Medic Pistol
2: RPG + M16
For option 2 you could make their argument of less raw damage for the ability to run faster by pulling the knife, giving the demo a tad more survivability if hit the fan or your temates re not up to scratch. But otherwise it begs the question, why not just go Berserker or demolition?