I have the Rocna 20 (44 lbs) on our 2005 400 and love it. I upgraded 2 years ago from the Delta that came with the boat (sorry, can't remember the weight) and much prefer it. It works great in the anchor roller, though there is not any room for a second anchor in the second slot.
I considered the next bigger size rocna, it would have worked too, but I was a little concerned about the strain on my 14 year old windlass motor, which burned out last month anyway (go figure) so may not have been my best decision. Still though, I think the 20 or the 25 is a good size for the MS 400. Rocna's sizing chart shows that both will work, just depends on how much weight you want to pull compared to the conditions you expect. The windlass burned up the first day of our three week Bahamas trip, and I had to hand pull for the rest of the trip. In that instance at least, I was glad to have the 20 and not the 25.
We use our boat in Florida and a lot in the Bahamas. I typically anchor in less than 20' of water, often in as little as six feet. I have 50 feet of 5/16 chain and 200 feet of brait. I'll likely add more chain someday, but this setup works great for me now.
We use our Fortress FX 55 as a second anchor. I store it in the anchor locker (it's a tight fit) and hand launch it, which sucks. I have 25 feet of 5/16 chain and 150' of line on it. I used to have 70' of bigger chain on it, but reduced the size and weight of the chain because it was too heavy to hand pull easily and didn't fit my windlass gypsy.
Hope this helps.