Why not get both?, the 13X26 for longer tougher hills and/or early season and the 11X23 for when you're in better shape or for flatter rides? Is there a 12 or 13X23 available? Very, very few people are strong enough to really need/use an 11, and you'd gain a more usable gear in the middle of the cassette you might miss with an 11.
I have- 13X21, 13X23, 12X25 and a 13X26 (8-spds) for my main ride and use them all! and I wish I could still get a 12X19, 21 and 23. I'd only need the 12 on some sprints, to "keep-up" on fast descents or time-trials with some downhill grades.
When I get bike with a 10 spd set- up, I"ll get a group of casettes for it too!
My faves are the 13X23 and 13X26 in my present fatness.
(No, you don't have to change chain length for them)

Ride Many Gears
Pat