I just installed (myself) the BMW hitch on my 2012 X3 M Sport.
The Invisihitch looks really nice too. I would not, however, leave the gravel guard off because it attaches to the bumper cover and it looks to me like it adds some strength and integrity to the lower edge of the cover.
I cut out a section of the gravel guard for the BMW hitch and this could be done for the Invisihitch also.
When it comes time to return your leased X3 you could simply take either hitch off and put the "cut" gravel guard back on since you can't see that it's been cut unless you get on the ground and look up, or just buy a new gravel guard from BMW and put that on. They can't be that expensive. Certainly a lot less than a new bumper cover.
Just my 2 cents worth!