The 28 cog is good on moderate to somewhat steep hills. I see you mentioned you will be doing hills occasionally.
For mostly flat ground, a 12-25 is really nice. (But hills will be harder!)
Here's a 50/34 and 12-25 in 11 speed. See that there are much closer shifts in the 15-25 mph range, but you lose the hill climbing gears, of course.
(It's easy to switch cassettes, you need a cassette tool and a chain whip. I swapped out my 12-29 for a 12-25 for my quite flat ride tomorrow.)
Last edited by rm -rf; 02-18-17 at 07:47 PM.