I own the RIA Ultra FSHC or whatever they call the $650 double stack 10. The 'standard' dust cover with no tac rail. I also have the Witness Hunter, which I've shot a little bit more than the Rock. I'll just share my experiences with each, but first, about utility, use, etc.
I don't hunt and never will, so neither pistol was purchased for anything other than range toy. Neither is well-suited for IDPA or any other kind of action games, IMO, so again--strictly guns for taping up paper and setting up pumpkins and blasting away, for ME.
I feel both are equally well-made, all-steel guns. The Witness is very 1911-ish in some ways, but obviously so many differences most folks would take issue with that. They are both relatively heavy steel guns, so both help manage recoil, although the Witness is to me considerably more comfortable to shoot. It has a grip that is barely wider than a standard single stack 1911, has no sharp corners or edges anywhere, and simply feels very nice in the hand.
Until I benched out all the skin-ripping corners everywhere on the RIA, it was literally painful to shoot. About 30 min work tops with a die maker's riffler (jewelers file), it is very comfy, and I can shoot it extensively with no issues. It is still a comfortable grip, although just a little fat. I find it far more comfortable than say a G20. No contest--for ME.
Adjustable rear sights on both, similar takedown, nicer finish on the Witness if you consider 'nicer' to be shiny and classic. I personally love the plain, matte dark finish of the Rock, and particularly like the fact there isn't any goofy crap stamped or engraved all over the gun. The EAA gets points for this as well--nothing I dislike more than script lettering, logos and safety instructions slapped all over a gun.
To the very best of my knowledge, pretty much all aspects of the RIA are standard 1911 A2...give or take a trigger part or two and maybe the recoil spring and guide rod assembly. I'm no expert in that area. Nothing on the Witness is 1911, but parts are available just the same.
The windage adjustment screw on the rear sight of my Hunter has always had an issue with backing out over extended shooting (or one of the screws, anyway), while the fiber element in the fiber optic front sight on the Rock dropped out after a hundred rounds. Both of these issues are minor and correctable. The only other issue that comes to mind in the guide rod in the Witness is a two piece thing, the two pieces pinned together with a small steel pin. That pin sheared and the two pieces came apart, with the guide rod itself falling out of the gun at the range. EAA sent a complete new guide rod in a couple of days--but I simply took the two pieces of the first one and fused them together permanently with JB Weld--and it worked with no issues for maybe 1000 rounds so far. The two-piece design is, as far as I can tell, simply a manufacturing choice to get a fat, load-bearing end at the rear where it contacts the frame, and a long slender spring guide--without turning down a solid piece of round stock. There isn't any reason the two pieces ever need to be disassembled--so now they are as one. I would have TIG'd em if I still had my welder.
So, I give the nod to the Witness Hunter for overall comfort, but the RIA gets the vote for 'standard' and things like it slaps into my 1911 holsters like they were built for it--which, I guess they were. I doubt the extra length of the Witness is going to add 50 fps to muzzle velocity, and since I handload, I don't care about that anyway.
Speaking of handloading--my choice of calibers and weapons, as a handloader, can be significantly different than those of shooter who doesn't choose to handload. I don't shoot factory ammo...period. So, 10mm is my choice of handgun caliber (for auto pistols) and it's not very likely I'll ever buy a firearm in any other caliber--10mm is about 6 cents a round for me if I shoot cast, of 35 cents a round if I shoot jacketed. Same as 45, same as 44 Mag, same as 40 SW. Cost per round isn't a factor.
As with 45 Auto, 44 Mag, 40SW and every other caliber than can be handloaded, 10mm can also be loaded to a *range* of loads. This may come as a real shock to many shooters, because I seem to hear the opinion (a lot) that there are only two choices in 10mm Auto...'real' 10mm, and 'not real' 10mm. No other cartridge ever developed, that I'm aware of, had only these two choices. As far as I know, there is no 'real 44 Mag' or 'real 45 Auto'. But, I've discovered that 10mm can also be loaded to supremely accurate target shooting levels (sort of like 45, sort of like 44 Mag), still cycle a firearm, and still be a blast to shoot.
I think you'd be happy with either choice, I am.