I own both a Mark II and a Mark III.
I'd prefer the Mark III not have all the junk on them added since the Mark II, but don't feel that they detract so much that if a Mark II version of what I was looking for wasn't available and a Mark III version was, that I wouldn't buy the Mark III. My Mark III Competition is still incredibly accurate and has been very reliable. As far as details...
The new mag release would be neat if it didn't cause more problems than it solved. On Competition models, the thumb rest blocks it, and I've learned to wrap my fingers around the front to get at it. It also means II and III mags aren't interchangeable, so I had to get another set of those (I like to have five or more). Also, where the mag release was on the II, is just a block of plastic Ruger used to fill in that spot. :/
Mag safety I couldn't care about, since this is a range gun, but you now need to have a mag inserted for certain steps of field stripping.
Loaded chamber indicator is fine, except that it is made of plastic, and the metal extension in the receiver makes it difficult to get at spots when cleaning the breech.
My least favorite "upgrade" is the tapered bolts ears though. During extended shooting sessions they seem to cut into my fingers more than the old untapered ears.
That's a lot of gripes but I still love it.