I live here in Rifle. I hit Harvey for Pike. You'll catch TONS of perch in there (50+ a day between two of us), but your average is gonna be <6" most of the time. You'll pull out 4" females full of eggs. It is WAY over populated with perch. About once a season, we get something that hits 10" to 12", but they are rare. Rifle Gap, as stated above, is better quality perch, but FAR less numbers.
For perch, pick your poison.
I know you asked specifically about perch, but if someone else sees this and wants to know about other species, I'll list the following...
If trout is your game, there are better trout in Rifle Gap.
Pike is a toss up. I've caught 36"+ pike in both places. Be CERTAIN you have a pike if you catch it in Harvey and want to keep it. There are musky in there that can't be taken out.
Crappie and bluegill are usually better for us in Harvey.
Walleye are in Rifle Gap. Some (2 guys) claim they have seen walleye come out of Harvey, but I've never seen proof, or talked with anyone else that has either.