I have a road bike with a 26/11 9 speed SRAM rear cassette. I cycle in Cambridgeshire on a daily basis and it's very flat. I find that I never use the hardest cog, leave the chainring on the biggest cog and generally find that the jump between gears is a little too much.
My chainring is a compact 50/34 110mm.
Sometimes it gets very windy and I'm forced to have to change to the smaller chainring cog - I guess that means I don't want the big cassette cog to be any less than 26 or so. I'm not a fan of changing between my chainring cog much as it kind of changes everything.
Ideally if it exists, I guess a 27/14 (do they exist?) or something similar would suit me better... not sure if 27/15 would cut my top speed down too much though.. I find with my current 28/12 I can get over 30mph without spinning out (with lots of room to play). I never go this fast unless I get an amazingly lucky tailwind :) There's no downhill here either!
Basically I have no experience of different cassettes so any guidance from an experienced cyclist would be of great help. I'd like to avoid buying something stupid.
---- UPDATE ---- In the end I replaced my 9 speed drivetrain with a tiagra 10 speed.. this was fairly involved as I needed quite a few new components: new shifters, cables, derailleur, cassette, also got new chainrings, crank arms, bottom bracket (and pedals as I just couldn't get them off!) One of my reasons for replacing all the old stuff was that it was worn, another was that I recently got a tax rebate!
The 'feel' of this change is apparent and it does seem to help a bit.. however speed wise my new wheels made the biggest difference!