I had a Snap Duo briefly and thought it was pretty great. I think it's the lightest or one of the lightest doubles out there. If you ever fly American Airlines and plan to take your double with you... you want this stroller because they have a very strict 20 lb weight limit on stroller. Now the Snap Duo is 22 lbs but you can take the wheels off and throw them in your carry on to bring it under the weight limit so you can gate check. Snap Duo has a single canopy which you may or may not like.
The Zee Two is just more featureful that the Snap Duo. The basket on both strollers are pretty great too. I would probably pick the Zee Two over the B-Agile double just for the basket. The Snap Duo handlebar isn't adjustable but the new 2013 Zee Two is so that might be important to you. Any of those are a nice choice. I like the look of the Valco strollers more than the B-Agile but that's purely personal preference. I know that that Valco strollers can take a Graco car seat also. I *think* that the B-Agile double only takes a Britax car seat but I could be wrong.
Edit - it does seem that you can work around to make a Graco car seat work with the B-Agile. See this thread
http://www.amazon.com/review/REQES7F...#REQES7FS8D4FX