Primordius is way easier if you're geared. It's pretty much just a gear check with loads of numbers, some tank dancing, and some spreading out/soaking. If you're undergeared, he might present a pretty serious challenge.
Durumu is harder executionally and numbers still matter. Everyone in your raid needs to know how to handle life drains, pilot beams, and do the maze. You also need to put up some hardcore numbers, moreso if you don't have an MS. Also, our BrM solo tanked. You just need a single paladin in the raid to solo tank.
We got Durumu down in 1 night (4 hours) total, on our last pull. Had a 500k wipe after 2 hours where we tried to burn at the start of the 3rd light spectrum phase, but a life drain came. It's really not that hard depending on your comp. Ele shaman melts the ice walls so they weren't a problem at all for us. We were dispelling the DoT pretty early and the eyeballs didn't seem to do anything. Tank damage is really high, but if you can manage to solo tank it the DPS pay off will be worth it. If you can do the maze on LFR, you can do it on heroic. Only hard part was the 2nd spectrum phase where you get life drain during it. Marking adds and using defensives to get it over fast is the key. With average DPS if you don't mess the drains up you can burn the boss at the start of the 3rd spectrum phase.
Haven't tried Primordius yet, but expecting it to take a few hours (double Destro lock)
My guild killed Heroic Prim tonight in about 2 hours on our first night of attempts. We had a 5% wipe on our second attempt EVER. Its a very easy fight. Gear is not what made it easy for us also. We are a Top 50 US guild but our ilvl is outside the top 125.
Durumu wasn't as bad as we thought it was going to be. Bring some Warlocks (surprise!) and Elemental Shamans. Then you just need to set up spots for beam placement, and pay attention when the Fogs are shown briefly.
Primordius is more straight forward, but still both require a solid strategy.