For a little while, at least, a hardtail will make you a better rider. It will teach you how to level out your pedals on the downhills or swoops, switch gears at the right time, steer tightly, use your legs as shocks. Once you've switched to clipless, a whole new world, and gotten to use those as tools for improving your bunny hopping and pedal stroke efficciency, you might be done with the old bike.

Eventually, when you stop improving and/or if you get OLD ( says the almost-33-year-old with the "some dehydration" in her spine discs) a rear suspension can make a lot of the bumps disappear and float right over things that the hardtail's rear wheel used to clunk on and jar the he!! out of you on.

My Giant Trance is really such a fantastic improvement on singletrack from my old beloved Trek 8500 that it's not even quite the same thing. But I'm glad that I learned a lot from my old bike before it was time to retire it after five years and turn it into a commuter.

IMHO, as long as I have room to store it, I don't ever want to sell or trade in a bike as long as I have some excuse or niche in my stable for it. Keep it, it's an option in case your new bike implodes and you need to go back to it. But new bikes are good for the economy, the motivation, etc. That being said, I feel your pain when it comes to paying retail so I would always test ride something until you love it and then check eBay or online classifieds until you can afford the price.

Good luck!