I am not sure what you mean about "missing something". Is this a matter of deciding which one to get rid of? Obviously there are quite a few of years separating the two so I always keep a historical perspective when evaluating them. Otherwise, they are both excellent light tight boxes.
As a continuation of cooperation between Leica and Minolta, the XD-11 is obviously a well designed and made product. The FM3A's hybrid shutter was unprecedented and currently unsurpassed. You can go down a long list of feature differences between the two - shutter, aperture and program mode auto exposure, batteryless manual speeds, interchangeable screens, eyepiece shutter, auto MLU, DX decoding, TTL flash, safe load indicator, multi exposure lever, etc. However for me, there is one thing the XD-11 does that always nagged at me and that is the momentary lag between pressing the shutter and it's firing. This happens even in the manual mode at 1/100 and has been observed and reported by others. Not that I've missed a shot because of it but just that it does. This seems to be unique to the XD-11 as my current preferred Minolta - it's predecessor the XE-7, doesn't do this and for that matter, none of my other Minolta MD's exhibit this trait.