Anything anyone is going to tell you will be personal preference without knowing you personally. Just go in to local stores, and demo demo demo.

I second the, try the bikes out and look at their demos for sure!

Step 1- find a good store with an experienced tri-bike fitter (ST has a FIST database to aid in this). You may need to travel
Step 2- schedule a fitting. There will be a fee, which most likely will be waived if you purchase a bike from that shop
Step 3- learn which tri-bikes actually fit your body type. They are not all created the same. Plus, each bike manufacturer has a different system for labeling the sizes on their bikes.
Step 3.5- If in doubt, get your 'stack and reach' numbers from the fitting, start a new thread called "What bikes will fit me?", include said numbers, and let the brethren help you. Slowman himself is likely to chime in since that's his thing
Step 4- get on a few of those bike and test ride them. Try and feel the difference between a low level, mid level, high level ride. Be patient and visit a couple of other shops should the place you got fit not have all of the options.
Step 5- Remember that you have to make other decisions than just the frame. You need to know if and how much you will accessorize. At the minimum, you'll want pedals and shoes. Do you like the saddle? If not, exchange it immediately and negotiate a trial where you can return a saddle after a trial period. You'll probably also want some sort of data device. GPS? Do you want to train with cadence? This is an add-on. How about power? Another add on. How are you attaching this computer thingy to your bike? How about a flat kit and where are you storing it on the bike?
Step 6- Actually buy the bike.

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Take a short break from ST and read my blog:
http://tri-banter.blogspot.com/

I'm beginning to understand this is no simple clear cut purchase, some great advice very much appreciated.
I'll be sure to start researching now and develop more understanding of bike particulars and individual preferences I may have before heading to the store.
Looking forward to the new gear and regular training.
Many thanks!

The advice given so far has been pretty good, but a bit abstract. If you want to start thinking about bikes that are in the mid-high range, then these articles by Dan Empfield (Slowman) are useful, because they define what makes a "good" tri bike in 2015.

http://www.slowtwitch.com/...Sweet_Spot_5289.html
http://www.slowtwitch.com/...Spot_Bikes_5291.html

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'It never gets easier, you just get crazier.'