I rode a 12/25 in and around the Atlanta area (pretty rolling terrain, some steep stuff, but not very long at all, I'd say everything under 1 mile), but when I first went into the mountains a little farther north of here (not much steeper, but much longer climbs, up to 8 miles, I used a 12/27. The bailout gear came in handy a few times as I tried to haul my carcass around a few of the switchbacks on Wolfpen. I'd say that if you hardly ever use your 39-25 now, but you think you might need a bailout, the 12/27 will work just fine. The cassette is hardly different at all, except for the last 2 cogs--a 24 and a 27.