It would kinda be noticeable. In real life the new 2010 Camaro is 2.8 inches longer than the newest Mustang, so if you had:
1/24 Mustang - 7.8 inches long
1/25 Camaro - 7.6 inches long.
So yeah, while the real Camaro would be slightly longer, the models would be the opposite. But then again its only .2 inches, and you'd have to have a really good eye to notice. If you see the two you want and they're diff scales, I'd still go for it unless it was somthing odd scale like 1/20 haha.
And even then, it also comes down to how you pose them together, because I don't really think that 0.2 inches will really be noticable unless you put them super close together like side against side. Just pull out a ruler and see how small 2/10ths of an inch is.
As far as the difference.. dunno, but I wish they would just make ONE. Because it seems to cause a lot of problems, whether it be causing questions like this or even decals. I swear some of these 1/25 Nascars I've put together have decals for 1/25.. because even though its only like 0.1~0.2 inches different, it leaves so much overhang and its just wrong..
All model makers seem to do it though. Outta my collection of cars I have slightly more 1/25 than 1/24 but not that many and I have cars from just about everyone. The only one that seems pretty faithful to one is Tamiya, and they seem to do almost all 1/24, at least all the Tamiya cars I have are.