On my Giant, I carry a Blackburn with a swing out handle.
Small enough, light enough and it puts out some useful air.
I've actually used it and it works well.
On my Jamis, I carry a Road Mini-Morph.
I like it's a design and I just can't say enough about my Joe Blow Pro.
I like it enough to justify the Mini.
Never used the Mini, though, so I can't really say.
But, in any event, they're both just insurance because my CO2 is my first line of defense.
I carry it, loaded, and a spare cartridge.