I think the rest of it boils down to personal preference. Strongly suggest you go take a couple of test rides, and see if YOU like the bar-ends, or prefer STI. If you want STI, the laundry list Russ suggests is something you'll have to work out with your bike shop.
Just to recap, on the Opus you'll have to swap out the inner chainring, the cassette, and probably add a chain watcher. You may also have to change the rear derailer.
Note, too, you'll still have a pretty big jump to the granny gear on the front (16 teeth). It would be nice to see how that setup works for you before you plunk down your cash or credit card. For all that work and extra hardware, most bike shops will want you to buy it before they make the swaps. If your LBS will let you try it first, they're exceptional and deserve your business.
With all those changes, I'd probably buy the Trek. But I'm not strongly biased towards STI.