I've cross posted this to the commuter forum, and a very similar question in the road forum, and I've even asked an LBS for their advice. I'd like yours as well, fellow Clydes and Athenas. There are a lot of smart people in this forum, and while it has less activity than other fora, there's a lot of quality in here. Plus, C&A is less specialized, and I'd like advice from some generalists.
I have a Novara Element CX bike; I got it used about 2.5 years ago. It's had some mechanical problems, a few of which nearly killed me. I don't really feel safe on it. Add in the fact that it's very heavy and doesn't handle well, plus it has mediocre components ... but it's strong and a jack of all trades. A bike shop offered me $500 for it. They have a used Cervelo Soloist, an alu frame with a carbon fork and Ultegra 6500. The shop said they're not sure which, but they could either pay half the sales tax or write the sale up as $500 + old bike. They also said I can keep my (SPD) pedals, aerobars, lights, and cyclocomputer, and get the trade-in. The Soloist is a 61 cm frame when I normally take 58 cm, but, with its aero geometry, it fits me well in this size.
My main road bike is a Cervelo R3, which I'm in love with. But it's carbon/kevlar, so I would never lock it up for fear of damage. This one is metal, so it would take over as my commuter, and my errands bike. Occasionally I like to ride out to a restaurant about 10 miles away, lock up outside, have a meal, and ride home through the city at night - the Soloist would have duties like this. It might also become my rain bike - I don't really like riding the Novara, so I take my R3 out in the rain, and it'd probably be better not to.
I'd give up off road riding, but I don't really do much of this anyway. There's almost nothing I love more than being in the mountains, but I learned I can do that on the road with my R3... I haven't really been going on trails more than a few times a year, and the Novara is such a harsh ride, I feel pretty beat up at the end of a decent length trail ride. But ... I have that "do I really want to give up being able to do this?" concern.
So ... I'm not asking you what I should do, but I'm wondering if anybody has any insight, or if there's an angle I should consider but haven't. Stuff like that.