I thought I'd update everyone here and thank you all for the different viewpoints and information. I'm still new to quads, though I've taken every opportunity to fly in the last month or so, so take this all with a grain of salt.
After flying a few different Revs, I settled on a B1.5 mid-vent... and couldn't resist a B1.5 full sail to go with it. I discussed options with a couple stores on the phone (thanks to all) but wound up buying from my semi-local shop (Cath and Eliot at Flying Smiles -- thanks for discussion and package deal). I probably couldn't tell the difference between the 1.5 SLE and the B1.5, but based on recommendations from local Rev flyers, I went ahead and invested in the Bs. The price difference wasn't that big, and I didn't need the SLE frame. There were times in the last month that I should have been flying a full vent -- I admit to foolishly risking the mid-vent in winds much too strong for it last week -- but that's very unusual where I live, so I think the mid-vent will get more use in the long term. In my experience, I find the mid-vent tough to fly when the wind lulls below 10 mph, but again, I'm still new to quads, and I'm usually using a 3 or 4-wrap frame to deal with the gusty spring winds. Even though the kite stays in the air and has a little forward power at 10 mph, precision flying is hard because (for example) I can't help losing altitude at each clockwork turn, and inverted hovers are not yet doable. And as for the full sail, with 2-wrap frame and 50-pound lines, at 3-4 mph? No way! Not yet, anyway.
That brings me back to my original question about wind ranges. I've often thought that dual-line kites should be listed with more info about their wind ranges. Most have a minimum (kite stays in the air only if the pilot provides the wind) and maximum (kite barely escapes being damaged) wind range, but if I made the rules, they would also be listed with a range of no more than two or three mph at which the kite is at its very best. That could be useful information, but then (as many have pointed out), wind quality and pilot skill add additional variables and my idea falls apart. For quads, I think peak performance could be wider than that 2-3 mph range, but not so wide that we're talking about beginners struggling with low winds or flying with a lot of power in the sail. Personally (again, I'm a beginner with a humble opinion), the mid-vent performs well from 12 mph up to about 18, whatever the frame choice.
I don't have a well-formed opinion on the full sail because I still usually reach for duals in winds up to 10 mph. That second B was a big investment, but I'll get my money's worth over time (although the same line of reason is telling me to pick up a full vent to be prepared for the next unusually windy day). I'm planning to try stacking them, too. I ordered some line and sleeving from our friendly forum sponsor two weeks ago and it hasn't shown up yet; the tracking number is still shown as "Electronic Shipping Info Received" -- I need to call Steve and ask what's up.
And to complete the cycle, I gave an onlooker a quick lesson today before the rain chased us off the field! So that's my story of getting into quads. :-)
Update 4/24/2011: I'm being too hard on the mid-vent's low-wind capabilities. Had a good session with it yesterday in winds of about 8-10 mph with 2-wrap frame. I still wouldn't say that I can make it perform well at that wind speed because precision maneuvers are worlds harder than at 12-15 mph, but it's definitely flyable. I assumed that most of my dual-line low-wind skills would transfer to quads, but I suppose it's a different skill set.
Also, my stacking line and lineset material is still lost in the mail one month after it went in the mail. Happened to me the last time that I bought kite parts from the west coast -- how do packages get lost in the age of tracking numbers and instant on-delivery notifications?