I also own both, and like both. I carry each one as well. Both are highly accurate.
The Dan Wesson guardian has the aluminum frame and is lighter. True commander size with a bobtail.
The STI guardian is a CCO size, not commander size.....4" barrel on an officer size frame. Steel frame soaks up recoil nicely, but it is shorter. If you have big hands, keep this in mind.
If you want to compare apples to apples, you should be looking at the STI Ranger....that is a commander size gun.
Life is too short to not buy the handguns you want.....or think you gotta have.......or is just pretty......or a good deal.......(insert reason here)