The Rocna was one of our first upgrades on the R-31. I sailed with a Bruce for many years in the PNW, but I think the factory anchor is a wee bit undersized especially for short-scope anchorages, and my co-owners were anchoring novices. We've been happy with it.
I have never found the need for a swivel and literally every bone in my 35+ years of boating experience body says do not get one. Talk to any of the old salt Caribbean cruisers and they'll tell you that a swivel is a great way to lose your boat. The Mantus swivel may be the only exception, so if you see "swivel" settle for nothing less than Mantus.
That said, I've found that the ability of Rocna (and even Bruce/Lewmar Bruce knockoff) anchors to hold and re-plant in Northwest anchorages to obviate the need for a swivel. I think it provides peace of mind more than anything practical.
Remember that if you are going with the Mantus swivel, inspect it at least annually as you do with your chain, with a quick inspect every time you pull up from anchor. The breaking strength of the Mantus is based on factory-new condition, any swivel (even an over-engineered one like the Mantus) won't take a beating quite as well as chain does. And the swivel is at the "beating" end of your anchoring apparatus. Over time it might become the weakest link in your setup, even if it isn't at first.
And don't skimp on the safety wiring / mousing all shackles. If you do a casual inspection in a random marina, you might be surprised by the number of shackles that are improperly fastened.
