What's your budget? You say you can't spend very much, but didn't say how much you could spend. If you can spend $200 or so check out the Vortex Viper 2-7x33 at Cameralandny on clearance for $175, or the Nikon Monarch UCC 3-9x40 that's on sale at several places for $200. These are the two best scopes in this price range.
If you don't want of those, this is a great read on scopes under $200
http://opticstalk.com/inexpensive-sc...opic21176.html
That test didn't include the the two mentioned above as they weren't on sale in that price range at the time it was written. They are both a step above the ones in that test.
Another thing to mention is that while I have a Prostaff and my findings are very similar to in that thread, they are on sale at many places for $100. While it's no where near as nice as the Burris FFII I have, it's pretty good for the $100.
I don't have a Diamondback, but I do have a FFII and I absolutely love it. It's great in low light and in the daytime. Just a great scope.
I've also had a couple Leupold VX-I's and both failed and quit holding zero. The glass was also worse than the Prostaff. Seemed similar in the day, but in low light it wasn't quite as bright as the Prostaff. I actually tested it against a $30 Tasco and $40 Simmons and couldn't tell a difference between it and the Tasco. The Simmons was slightly better in low light. Not that the Tasco and Simmons are good, just that the Leupold is bad especially for what it costs. The only good thing is they do have a great lifetime warranty. The Rifleman line suggested above is even lower on the totem pole than the VX-I, so now way I'd ever recommend one of those. If I was going to get a Leupold it would be a VX-II at minimum and even then you can get a lot better glass for the same money with other scope companies.