Join Date: Jan 2008
Location: Beaverton, OR
Bikes: 2013 Kona Jake, 2015 Kona Jake the Snake, 2008 Kona Major Jake, 2013 Kona Jake the Snake, 2006 Kona Kula, 2012 Ridley Excalibur, 2001 LeMond Buenos Aires, 1984 Pinarello Gran Turismo, 1982 Trek 614, 1978 Austro-Daimler, 1987 Pinarello Montello
Mentioned: 23 Post(s)
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Definitely, use the Diamondback. Anything you get cheap will be heavy, but your fitness and handling skills will have a much bigger impact on how you do. Use the MTB for now, and when you start moving up in the standings, then you can drop some change on a good CX bike. No sense spending money now on something else that you'd want to replace anyway.
If it's got a rigid fork and a relatively flat top tube you're golden. If not, that's OK too. Also, I wouldn't worry too much about CX specific tires unless the courses in your area are very flat and fast.