The 4300 is a half step above the 3900 in Trek's line of bikes. Looking at their website, the components are largely comparable, but the suggested MSRP is $110 more for the 4300.
If they're asking the same price for both, I suspect the 4300 might be last year's model. Which makes it a better deal. In your shoes I would go with the 4300. Looks don't really matter, unless you're just buying the bike to cruise around town and pick up chicks. You'll need a pretty big basket for that.
The 4300 and 3900 mountain bikes from Trek are equipped with a set of standard sized 26-inch tires that can adapt to numerous trail conditions. While the Trek 4300 is equipped with a conventional cantilever brake system, Both the 4300 and 3900 feature a suspension fork that allows the front wheel up and down while riding over bumps in the trail.
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