Garmin 910xt vs edge 500

I have a 910, and love it.  It is great for multisport.  With that said, I am considering getting a 500 as well for the bike portion.  I keep my watch on my wrist when racing (on bike when training.  For training it is fine, but I don't trust the QR with all the threads about people losing their watches, so I would like an ant+ ready bike computer.  As far as being able to see the watch, there is no problem.  You can set it up to have 1,2,3,4 diferent metrics, and you have up to 4 different screens to watch so really up to 16 fields available.

No real suggestion here, but from my experience a dedicated bike computer would be nice.

As far as visibility on the bike goes, I haven't had any problems with the 910xt.  My aerobar setup doesn't leave a lot of room for the QR mount, so I opted to just rotate the watch band so that the face is on the inside of my wrist.  This makes it MUCH easier to check while aero than my old 305 in the QR mount.  It also ensures that I don't forget my head unit on the bike during T2 (not that I've ever had a problem with that)

As to the screens, like velocomp said, there are plenty of options.  Though, I don't like to toggle screens while riding.  I just set one screen up with the 3 or 4 most important metrics - in my case, HR, Cadence, Time and Distance - and maybe take a glance at the other screens when stopped. 

So i shouldn't be worried about not able to read it while on my bike but more afraid about losing it cause of the quick release... At 400$, you don't want to hear that

I have both.  I had the Edge 500 before I got the 910xt.  Personally I really like the Edge 500 better for cycling.  It is easier to see and the customization is better. I want 5 pieces of information on my screen (speed, cadence, HR, distance and elapsed time) so the edge is really the only way to go for that.  Even with 4 I don't think I would like the 910xt screen b/c everything is the same size.  I like the fact that I can have one section bigger than the rest on the Edge with 5 pieces of info (b/c like you I don't like scrolling while on the bike).

I really don't like the quick release on the 910xt.  That said if you can only have 1 and you are going to do multisport then get the 910xt because of the flexibility.  While I don't like it as much for purely cycling as my Edge 500, if I could only have 1 I would want the 910xt for the running and the swimming.  However, if you can have both and/or are happy with your current solution for swimming and running then get the Edge 500.  It is a better cycling option IMO than the 910xt.   When I just ride I never use the 910xt and when I compete or do brick workouts I use both the 910xt and the Edge 500. 

Some pros/cons of each...The Edge also has better battery life IMO than the 910xt.  Screen is easier to read on the edge and there are more mounting options IMO.  The size of the Edge can be an advantage also.  However, the wireless communication of the 910xt make it more user friendly for Garmin Connect and TrainingPeaks.  The 910xt is more flexible and useful over more sports.  Satellite pick up is quicker on Edge. 

on race day, it's nice to have one device that does everything.

but for every other day, I prefer having a dedicated device.  I think you'll find that the edge 500 is a better bike computer than the 910xt. 

If you already have a running watch, like one of the forerunners, and an edge 500, then I'd say it's not worth the $ for a 910xt. 

My vote- just get your wife a new bike computer.  Plus- giving her your old computer and buying something new for yourself... may or may not go over well with the Mrs.  your call there.