Comparing the two is like comparing apples and oranges. Both are outstanding but very different. I have both. Here is my take...

Mustang: Outstanding, scale looking airplane. It flys like a Mustang... relatively heavy wing loading, super stable, flys like it is on a rail, needs a little speed on landing. It can do basic aerobatics similar to a full scale mustang such as loops, point roles, inverted flight, stall turns, etc. I personally like doing high speed, low passes with it; warbirds just seem to be perfect for that. The biggest drawbacks to the airplane in my opinion would be poor ground handling compared to some other planes (typical of a warbird), needs speed on landing or it could tip stall (also typical of many warbirds, especially Mustangs), and problems with the landing gear (another common warbird issue). These negatives aren't really that bad though; they just come with scale airplanes of this type and they aren't that big a deal if you are ready for them. If you like warbirds and specifically P-51's this is an excellent airplane.

Extra: This is another great airplane. It is much friendlier than the Cap-580 in my opinion with a lot less tendency to tip stall at slower speeds. That doesn't mean that it won't but it is more difficult to do it. The plane is very stable, failry sedate on low rates and EXTREMELY maneuverable on high rates. It can do 3D stuff with enough power but won't be quite as good of a 3D machine as somethign like a lightweight profile. It flys fairly "neutral" with only some slight coupling issues when rudder is applied. Knife-edges, flat spins, point rolls, blenders, etc. are all easy and nice to look at. Landings are almost automatic with this airplane. Drawbacks? Can't really think of any. I'd like to have a slightly bigger one but then it wouldn't be a 40 size plane either.

I think it bascially comes down to two things: What kind of airplane do you want and where is your skill level? Both are outstanding.