If you're ever going to use the bike for cyclocross, the adjustable stem may be risky, but otherwise it's probably the way to go.

Short of paying for a full bike fit, it can be really hard to guess what size stem you're going to need. The adjustable stem isn't perfect -- it still limits you to a specific arc -- but it will definitely get you in the right balpark.

If you have the funds available, a bike fit may be worth the money. Sunset Cycles does a "comfort" fit for $45 + parts, which sounds like all you'd need.