Went with the matching vest. It wasn't the most popular option here, but I felt like wearing a vest.
I'm going to be the odd-one-out who says that the other one looked better, aren't I? I don't think that the green goes with the beige pants (or the tie) particularly well. I much prefered the grey: subtle and more elegant, and the grey pants appeared to be slimmer and therefore fit better with the svelte cut of the jacket. But apparently I am in a minority here, so don't listen to me!
Edit: oops, just noticed that actually Cantabrigian has already said much the same.
Hmm. I guess I get what you guys are saying, and probably will still do the gray/green combo occasionally. But I am a little more comfortable in today's fit.
Yes, covert twill. Brooks Brothers sold them as cavalry twill, but that's a slightly different weave. I have them in gray and olive as well as tan, and love them.