Ohh twilight zone time. I had a customer (female, 50s) just a few weeks ago looking at those exact two bikes!

She kept walking over to the 2100 and picking it up and exclaiming "It's sooooo light".

Sad, but true, the components that help make the 1800c comfy (suspension seatpost and adjustable stem) also add weight.

Your Trek dealer should be able to order those items for you if you want to add them to the 2100.

What we did for our customer? Made sure the top tube length on the 2100 was appropriate for her. Ordered and installed a shorter stem with more rise (we went with non-adjustable to shave weight; once you have fit dialed in on a road bike you really won't be messing with it too much) and rotated her handlebars ever-so-slightly upwards. She opted to skip the suspension seat post as she found that the carbon chainstays provided enough comfort.

What should you do? Go with the bike that feels best to you and best suits your needs. Our customer was planning to ride ~50 mile club rides and was on her second (perhaps third...) road bike. She wanted lightweight.

If your main goal is comfort and you plan to go with a suspension seatpost and adjustable stem anyway, I'd probably urge you to go with the 1800c.

As far as the 105 vs. Ultegra debate, the newer 105 is fine. I used to say save up for Ultegra if you plan to race, but I'm seeing 105 on beginner race bikes these days. It's neat how technology trickles down!

Gearing: try to test ride the bike up a few hills. If you need a larger (easier) cog on the back you should be able to get a 25 or 27 with out toooo much trouble. You could also have a smaller small chainring installed. Are you able to get up hills in your area on your mtb in the middle chainring (usually a 32)? You could also ask friends with similar abilities what gearing they run.

You're lucky to live in an area with such beautiful terrain (My daughter will be going to school in Pittsburgh this fall and I plan to visit--and ride!-- often )

Veronica: I moved from Austin TX where every bikeshop had at least one female employee. At the beginning of this season I attended the Trek/Gary Fisher/Lemond tech session (for shop employees) in Indianapolis....and I was the only female in the room. Yikes!