As there's no specification for 2.5 gigabit ethernet (that I'm aware
of), and SONET gear with pluggable optics is likely out of your
league, I'm afraid that's a decision you'll have to make. :-)
There are plenty affordable[1] options available 1U for switches with
10GE uplink ports. I've had good luck with the Extreme Summit x450.
If you're a bit more daring, Allied Telesyn recently announced a
switch with a similar port density, though lacking in the useful
"bunch of SFPs rather than 10/100/1000 ports" configuration.
More and more, I'm finding it difficult to justify deploying n x GE
port-channels across dark fiber, when 10GE is a lot easier to scale
and troubleshoot...
Since we are clarifying...
Yes, 10G is a nice option when you can use reasonably priced optics
(<80km). CWDM (nx2.5 or nx1G) works happily up to 30+db of loss and
reduces complexity if you can avoid all the associated pieces of DWDM
you may need to address conditions in the long reach end of the range.
Deepak