I just realized the one bit of info I left out when I posed my question --

It's my husband who asked it. Or rather, has asked me, "Why would you get a lighter bike? If you're wanting a workout, you need to work harder, not easier."

I want a lighter bike because it will be more fun to ride. And I'm sure I'll ride more when I have a lighter one. As to my goals -- I think I want to be a bit of a jock for the first time in my life. I'm not sure what that means. I don't have a specific weight or size I aspire to. I just want to be able to ride distances and be physically fit. I said when I got my bike last June that in a year, I planned to be in better shape than I was ten years ago. Well, that's hardly likely, ahem, but I've finally accepted that a lot of that is because of how hard it can be to ride my bike, which makes me less likely to take it out when conditions will make it harder.