I've done a bit of a SEARCH and the first thing I found was that there's probably an error in the model number for the second unit; instead of XQD161690LNF I think it should have been XQD101690LNF. If that is indeed the case then the only differences I can find are electronic and financial.
In general both units can be referred to as the Alpine R990 head unit, as fitted to the "high-line" ICE system. Physically, either unit should fit and connect properly to the CD auto-changer and the power amplifier. The front panel is identical, and so should the plugs at the back be too.
The 690LNF unit apparently has a "spatial feature" which I presume is something to do with the way that the stereo output is played, a kind of "3D Audio"; the 110LNF doesn't have this feature.
The financial difference is that the 690LNF is about 100 quid more than the 110LNF.
Both units are available on the (in)famous internet auction website.
And now, before you go searching fleabay, get yourself over to the "Introduce yourself" section; it's forum ettiquette before some of the other members jump on you.