What every body else said was good......and here are a few of my amateur observations.

In my opinion, it is very important to buy your first bike from a local reputable bike shop. You will need a lot of help with initial adjustments and you should have the convenience of being able to pop into the shop whenever to ask questions. Most bike shops will give you one year of free adjustments or some discount.

did you feel comfortable at that shop and the sales person? How does your bike friend feel about this shop? Of course the bike shop guy will be biased but take that into account and try to discern whether he is trying to get your the best fit. Maybe look to see whether he has the size bigger and smaller and ask to try them both too and ask why he feels the size is right. this can weed out whether he's just trying to sell you what he has. Ask if he will order the next size bigger/smaller, if you think you'd like to try it.

With just a basic look at your measurements.....at 5'7" and an inseam of 31" actually I'd say that your torso is on the longer side....My understanding of the WSD idea is that women usually have longer legs and shorter torsos than men. So it would be my guess that you would be ok on either. so I'd go with what felt best to you in terms of the handlebars.

materials......most everybody has a definate preference on this! but from what you describe your riding desires to be, I'd steer you to aluminum or carbon. If carbon is in your price range then I'd go with it. Carbon is going to give you an excellent ride and be lightweight. This is will be great for going fast in triathlons, climbing hills, and accelerating out of turns. Carbon will also be comfortable for long distance riding.

As to whether to go for the better bike as a beginner.....if you've got the money, then I'd say yes. since you are already athletic, you are just going to need to learn some of the more technical riding things and you will be going strong. You might be quickly disappointed with a lower end bike.

Keep in mind that as you get more addicted to this sport (and you will!) you will want more and better. One of the best ways to update any bike is to get better wheels. Rarely is a bike sold stock with wheels that are worthy of it. this cost could be $500 to $2000 in the next year or so.

hope this helps!