Specialized Tarmac vs Allez

Gotta ask. Went to the bike store yesterday and did a little test ride, or looked at the Specialized Allez. Well they had one my size got on and still couldn't get over the feel of a road bike. My bike now is a Specialized FSR which has to weigh at least 30 lbs. Anyways, the guy is showing me an assortment of bikes and was one heck of a great salesman, or at least he made me feel comfortable. Well the Allez felt right. Then he showed me a Tarmac which is one model year old and right now they were running a discount where I could get it for around 1700? Seems to be a pretty good deal and the bike looked great. Figure tht if the price of gas goes back up it'll be cheaper to ride the bike to work everyday and to get groceries, so maybe it'll offset it's self. (Me sad attempt of trying to rationalize the purchase ). Actually trying to just find some info out and it seems like all the sites I goto have great reviews for both. Want to do a little more research as i'm going to go buy one or the other next weke. They were both the same size and the Allez fit great. Didn't ride the Tarmac though. Can afford both but is there a website that will tell me what the differance is between double, triple, compact, etc. Sorry for this newbie question, just having a hard time finding the differances between all the differant designations on the models.