Go with the entry level 4500 over the 4300. Not the best all around for XC racing but it will get the job done. This one is a bit heavy for a first bike (around 28 lbs ) but good enough for a beginner like yourself, I assume. Don't know where to tell you to sign up for anything around you now. I was born and grew up in Waterloo but that was years ago and there used to be a race along the Wapsipinicon river some years ago but not sure that is taking place anymore up around Independence IA.
