Originally Posted by milkbaby

Smarter people than me will probably answer this better... The more relaxed geometry will usually have a longer wheelbase which tends to make the bike more stable but less twitchy and react slower to steering inputs.

The head tube will usually be taller, so this will affect how low you can position the handlebars and how low you can get for aggressive aerodynamic positioning.

The top tubes tend to be shorter and the head tube angle slacker, so these two factors tend to put the rider into a more upright position which many people find more comfortable than the prototypical long and low position of many aggressive race bikes. That also means your center of gravity will be higher, so it may be harder to aggressively carve corners and turns. The slack head tube angle also increases the trail which makes the bike more stable but with the same trade off of giving up quicker steering response.

Some of the "endurance" relaxed geo bikes also have additional features which are aimed to increase vertical compliance for greater comfort.

Which bike is faster for you may depend. Ride comfort is believed to be important in producing and maintaining power on the bike and may trump other factors such as better aerodynamics and quick handling. I have a CAAD9 which I have ridden comfortably for rides up to 100 miles, but I am very flexible and most of the roads near me are relatively smooth in good shape.

You should try to enjoy some extended test rides! Have fun!