Part of it is you're used to spin class, and part of it is your build. If you're female, test ride both "male" and "female" bikes. Women's specific design usually means the bike is meant for someone with long legs and a short torso. A men's bike is usually meant for someone with a long torso and short legs. In spin class, you're probably spending most of your time focusing on just spinning. On the road, it really helps to look around so you can handle traffic. You *may* find that you're happiest stretched out in spin class, and more upright on the road.

I would look at something like the Marin Fairfax or K2 Mach 1.0. One is a flat bar road bike (sometimes called a "fitness" or "hybrid" bike, or a rigid mountain bike... there's all kinds of names and you don't much care what it's called if the bike fits well). The other is a drop bar road bike. Both are fairly inexpensive, and should be pretty flexible. The Novara Transfer is a *really* good deal since it includes so many commuting goodies, and is pretty long in the top tube, so it might be a good compromise bike for you.

I would also test ride some more expensive bikes, and I'd test ride somewhere besides REI. REI is a great place to buy a bike if all you need for good fit is a saddle swap and maybe a different stem. But if you need more than that, you may be better off at a local shop that is willing to really swap stuff around for you.