I've been road biking for 2 years but now am looking to purchase a mountain bike. I looked for ones that cost less than $600 with no rear suspension and found these. Giant Yukon Trek 4900 Specialized Rockhopper Cannondale F300 Raleigh M80 I rented the F300 and it seemed like a nice bike, that time being the first time I 'mountain biked'(although when I was younger I had a BMX-type bike and was pretty fearless in the woods on it). However, these are all aluminum frames and I am am unsure about how well they hold up. Are there quality steel frame bikes in this price range? Is frame failure among Al bikes a problem, or am I worried about nothing? Any advise welcome. Thanks.