Need advice on components

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Howdy All,

Looking to buy my first "real" mtb to replace my department store Mongoose.
I've been looking at several bikes in the $400-500 range -- Specialized Rockhopper, Gary Fisher Marlin, Raleigh Mojave 5.5, Trek 4500.
My question is how do the Shimano components (mainly rear derailleurs and shifters) compare with SRAM. I've read enough to know there are some hardcore folks in each camp. What I'm really looking for is everyone's idea of of how the various levels in each line compare with the other brand. In other words, a shimano deore is better than a alivio which is better than an acera, but where would a SRAM X-9, SX5, and SX4 fit within the Shimano line up.

Thanx in advance for any guidance you can give this newbie.