Fifty Plus (50+) - 11-27 or 12-28?

I'm going to swap out the cassette on my Colnago Master XL to one with a wider range. I'll be taking it with me when I visit my brother in the next few weeks, and he has some pretty draining "rolling hills" where he lives. His suggestion is that I go with either an 11-27 or 11-28. When I talk to the mechanics at my LBS, they are not in agreement with which will function better with the Dura 7800 series drive train I currently have one it (Yes, I know it's a sin to hang Japanese parts on an Italian frame, but the DA is what was on it when I bought it.) One mechanic says the 7800 series rear dérailleur isn't really capable of dealing with a compact up front and a 28 tooth in the rear. The other says it should work fine. Any of you have experience with the 7800 series with a compact up front and 11-28 cassette?

I don't have experience, but saw this on Shimano's site:

NOTE: CS-7900 11-27 and 11-28 require the use of RD-7900 or RD-6700

One tooth at the the 27/28 end isn't a hill of beans difference. Any benefit is likely to be psychological.

I thought "rolling hills" was basically the geography of the entire state of PA. There's a flat part where you live?

I have one bike with DA 7800, Ultergra SL compact crank, that uses a SRAM 11-28 cassette just fine. The other bike had DI2 DA compact crank with a DA 11-28 cassette. Both work great. If you climb all the time, it's the gearing I would suggest. Unless of course you need a bike that has a mtb rear der. and a 11-32.

I got an 11-28 for the Horrible Hundred in November. I have DA brifters and Ultegra RD and had no problems.

Man, and here I thought 11-27 was Thanksgiving or something in 2011!

I've got a DA with a 12-28 and a compact crank and have never had any sort of shifting problems. Just remember to get on the 28 before you need it, not when you are desperate.

28T is right at the upper limit of the derailleur's capacity, so much depends on chain length, rear axle position (although you don't have any choice w/ vertical dropouts), and derailleur hanger tab length. Is 12-27 an option? I don't have any use for an 11T cog, even w/ a compact ringset, although some folks do.

I would use the 11-28 if I had any doubt. It should work fine. 28 vs 27 isn't a real big difference but it's a difference that could help a little.

I went with the 11-27, and took it for a shake-down ride this afternoon. It shifts just fine. Thanks for the replies.