DH did the talking for me since he had a lot of knowledge about components, etc....
Your LBS should ask you what your intentions are (centuries, recreation, racing, etc) and have a good idea of what fit you need by your build. The LBS looked at my build and immediately determined I would be best on WSD, then explained why this was my best choice. Our LBS let you take it for a short ride (although they may allow longer). They then put you up on your bike on a trainer and spent about 45 minutes getting everything where it should be. Make sure you wear (bring) cycling clothes. My advice would be to speak up. Think of it like buying a car, ask questions and don't let them pressure you. Hope that helps.