Trek Equinox 5 or Trek 1500?

It depends...but I think they may be on the small side for you. I'm 5'9.5", inseam of 34.5" and ride a 54 trek. My husband rode a 56, and is 6", but has a shorter inseam.

Again, it depends...but you should maybe try them first.

Especially in Trek. They measure to a different convention known as Center to top. Most measure center to center. Meaning a 54 Trek is more like a 52 or 53 in another manufacturer. And that bike is likely too small for you.

For a 70.3 comfort is king. It would be absolutely crazy buying a bike without knowing your fit first... yes, these are both quite inexpensive, but buying them now, selling them after they don't fit and you've been hurting for a while, buying something else, selling that one, buying a 3rd one and then hopefully getting closer to the fit you should've had in the first place... well, all of a sudden, those $250 (or whatever the going rate is in your area) doesn't look that bad, actually a pretty good financial decision at that point... In other words, get a bike fit FIRST then start looking for a bike.

In the meantime, think about how you are going to use this bike. Group rides, organized rides (centuries, charity rides, etc.), then a road bike. Only tri's and training mostly alone (or maybe with other triathletes), then a tri bike may be an option.

Height is a really poor indicator of fit... if you go to the extreme, most of the time you can use height as a "guestimation" that something won't fit, but not as to if it will fit. Case and point, very likely a 54cm Trek 1500 or E5 would be way too small for the average 6' person. BUT, potentially, there could be a 6' person that would fit perfect on them (although it's quite unlikely...). Factors that affect fit would be, upper leg length, flexibility, core strength, arm length, injuries/issues, terrain, types of races (length primarily), etc. etc. etc.

They're both probably too small for you. I'm 6'2" (not an accurate standard to base sizing on, but anyway . . .) and my fitted tri bike is a 58cm, my road bike is a 60cm, and my cyclocross bike is a 56cm. The cross bike is noticeably smaller and the fit is a bit awkward for me (but I picked it up for less than $400, so I can't complain at my own stupidity).

The 2007 is a 9 speed, 105-groupset bike, and 2006 is a 9 speed, 105/Tiagra groupset. Both are aluminum.

If you can't find a decent, 2012 or 2013, 10-speed aluminum road bike for around $1,000, you aren't looking hard enough, especially with winter closeouts. Also, your LBS will be able to give you a much better idea of fit, even without a full-on fit. I'm also going to presume you don't know a lot about bikes, since you're asking a fit question; that means the first stop after buying one of these bikes is probably your LBS, to have it looked at and tuned-up, so you're already in it for at least another $100. Add in a $40 chain, or a cassette, or cables and bar tape, and you're very close to the new bike price point, and all for tech that's 7 or 8 years old, plus all the unknowns of maintenance and the like. The $200 difference is well worth the piece of mind to get a professional to give you at least a cursory fit on a new bike. Especially when shopping at entry level, the price savings isn't, in my mind, enough to justify the risks to the uninitiated.

Personally, I think a first tri bike should be a road bike - it's almost as fast, and way more versatile, especially for group rides and the like. It's also generally going to be more comfortable initially, and if you don't like it, there is a larger resale market for road bikes than for tribikes.

This year, I bought my wife a Novara Carema Pro from REI - full 105 groupset, 10 speeds, aluminum frame, carbon fork, decent wheels and tires, initial fit and adjustment include, for around $900 out the door.