I'll sum it up with a great quote from Kevin Metcalfe:
"As for Cat 3 vs masters, I would wager that masters can be quite a bit faster. I would guess that in a cat 3 race the top quality isn't as high, but the depth of that quality is good so somebody will always be willing to have a go. In the masters you've got some legitimate cat 1's in the group so the top end quality is much higher, but the depth of quality is less. So when the masters race is "on" it will be going really fast and there may be less places to hide, but it may be "on" less often. The masters race will probably be more tactical also. Cat 3's as a group generally haven't figured out the tactics yet. There tends to be a lot of chasing."
This describes things as I've found them quite perfectly.
That was my experience in my first Masters road race, just last month. They know who the hitters are, so will happily chase them down - to the detriment of their own race.
In this instance, they let one or two riders scamper free - being Masters 1, they clearly didn't know those boys were strong and as I'd just won the National (Masters 1) TT they lined out on my wheel. I haven't had a race that frustrating in a long time.
Crafty old b@stards! Now means finding new ways to try and win... as at elite levels, there's normally the aforementioned depth that forces a lot more selections and won't leave strong riders as heavily marked.