Travel adds a different aspect to the pony. You have to take the reg off so the cylinder can be verified as empty (of anything, not just air). A lot of dive shops will be hesitant, or even refuse, to fill it. So the logical method is transfill from the rental tank, typically an AL80. Weight is also a consideration when you're taking your gear, not renting anything.
While there is a group of folks that think the SpareAir is a waste, I think any extra air is better than none. As others have pointed out, know your sac rate and understand the limitations of a small bottle. For me, it is strictly a bailout bottle. We don't push NDL limits. If it's possible, we'll be buddy breathing. If not, the 6cu bottle should get me to the surface, no SS, at a safe ascent rate, from 90'. If I had to start deeper, it should make the CESA start pretty close to the surface.
Whatever size you go with, the advice to practice should be heeded. If you need it, you want to be automatic in deployment.
H2Odyssey has a system that works on any size standard scuba tank (this is what I have). For travel, I take the 6cu. For local, I use the 13cu. If I had the other 3 pounds available under the 50 pound weight limit for the luggage, I'd take the 13cu (she usually uses that all up).
SpareAir only works on their tanks, currently only 1.7 and 3cu. However, according to SSI, they plan on having a 6cu out sometime this year.
Whatever you go with, don't ever think of it as a way to have more bottom time; it's a bailout bottle!
