I wouldn't be concerned either about it's being an ETA movement. I know a couple of people who have Cartiers with their version of the ETA movement. One in particular, says it is more accurate than the two Rolexes, with inhouse movements, he has owned.
I didn't realize, until I got a Cartier myself, and did more research about movements etc. Apparently, there are different grades of ETA movements, and Cartier uses some of the better ones, if not the best.
After my recent experiences with Cartier, I would not hesitate recommending them.
The Ballon Blue is a gorgeous watch. Your wife will love it! (and You as well

)
Cheers,
Carl