need help plz

I'd go the used route myself. With 500 @ this time of year you should be able to buy yourself a real nice ride. Get shimano xt or better. Hydro disc brakes is the way to go and IMO would rather have rim brakes than mechanical disc especially on a 29" wheel. Cannondales tend to have a compact feel that many like. Do your homework and try to ride as many types and brands as possible. Kona, trek, specialized, santa cruz, jamis. Think in general out of those mentioned jamis will give you the best bang for your buck as far as buying new. When you get on a bike that feels right you'll know it and that'll be the bike that finds itself a new home.
Welcome to the site.

check your local bike shop. they should have some of last years models brand new on sale for around your price.

theMeat makes some good points. Either of those would get you out on the trail, but you might do a little better with your budget. The c'dales have a longer top tube than many bikes. Best bet is to ride the bikes so you can see which is more comfortable.

Since the f7 was only a 500 bike new, it seems overpriced to me. If it's been ridden, the drvivetrain, tires, and brake pads will have some wear. All this adds up, and bikes generally are worth about half of the original price after a couple years - less if they need any work.

Mongoose otero super, you should be able to pick one up new for that on a 2008 model, I have one, and for the price it's excellent, slightly heavy but full suspension and easy to upgrade

i actually think i saw a used jamis forsale in my range ill double check....right now with the economy like it is im just trying to get a bike that will get me into the sport for a year or so just until i save enough to get a better one or mod the one i buy... i want disc brakes because ive heard there better in the mud and creeks and im riding in most places that have that... i dont wanna go with a full suspension ive rode one and dont like it... the diamond back is new but i dont know much about the components and my local bike shop one sales cannondale and diamondback the other doesnt really sell just fixes lol