Depends a lot on what the tank is doing. Ideally he should apply both major fracture from either puncture or morphs or noxious breath if he's DK, which takes 5K off the enemy armor and crushing which takes another 1K or so. You should be running sparpened weapons, which also take about 5K. Dungeon mob armor is ~18K so you would remove 10-11K off the bat. Kra'ag and Spriggan might be overkill for PvE and Red Mountain does better than Viper in PvE.
Okay, so ideally its neither of those...
I'm guessing you don't have vMA daggers and axes? If you do, then its obviously 5 TBS, 3 Vicious Ophidian and 2 Kra'gh (for single target) or 2 Velidreth (for AoE) and the vMA weapons. If you need more sustain (hard modes) use 5 VO instead of monster set.
If you don't, then in that case, I suggest you run 5 TBS, 5 Vicious Ophidian and 2 Kra'gh/Velidreth.
If you can't use VO, then there is one thing to consider: When you say PvE do you mean dungeons or do you mean trials?
If trials, run 5 TBS, 3 Agility, 2 Leki's and 2 Kra'gh/Velidreth.
If dungeons run 5 Night Mother's Gaze, 5 Red Mountain and 2 Kra'gh/Velidreth because you only have 4 people to debuff the enemy and not 12, so penetration becomes important. As someone already said, Red Mountain outperforms Viper in PvE.
But really, in trials you need vMA dual wield and bow AND VO in order to pull good DPS and be comparable to magicka classes. If you already have a magicka class that you are good at that, gear him up first, start doing some vet trials for that Vicious Ophidian and farm vMA for some weapons. Magicka DPS is just miles better without vMA weapons, and even with vMA weapons magicka is still probably better.
EDIT: didn't see your reply just above. Red Mountain offers arguably better stats than Viper and it will proc roughly every 2.5-3 seconds (as opposed to 4 sec from melee attacks only) which is why it is preferred. You're proc'ing Red Mountain off cool down as you have a *** of DoTs ticking at all times, so the thing is going to fire off like crazy, which will in turn out DPS the CP advantage that Viper has.