trek 1000 or trek 1500 - confused beginner

The main difference between the Trek 1000 and the Trek 1500 are the components used.  The Trek 1500 has much nicer components and the ones on the Trek 1000 are more entry level.  Personally, I would go with the 1500.  I was able to get mine on sale for $850.  It is definitely possible to get them cheaper. 

As far as the difference between the Treks and the Giants, a lot of that will come down to which bike is more comfortable for you.  Try all of the bikes you are considering buying and get the one in your price range that fits you the best.   

The 1500 has a higher level of componentry, mostly Shimano 105 versus Sora (or lower) on the 1000. The 100 is a perfectly serviceable ride, but as chirunner said, (within reason) always get as much bike as you can afford.

Do keep in mind that the bike is only a part of your outlay, especially with a beginner, as you will also need a helmet, pump, seat bag with tools, water bottles and cages, bike shorts, pedals and shoes, bike computer, and on and on. Make sure you have room in your budget for all that as well.

Like so?  http://newyork.craigslist.org/brk/bik/338595264.html