We've definitely thought about the duallie... it's even possible my SIL would give me theirs (assuming they still have it?) since their youngest will be 5 when DD is born. But I would *never* want to travel with it. No way!!! We also don't have tons of space since we still live in an apartment. DH and I will mostly be together when there's occasion to be with both kids, but I'm sure I'll need to walk to the grocery store or similar with both.
Whatever double we get, I'd almost certainly get off of CL and then re-sell or gift when we're done.
My kids are closer together (19 months apart) but we still use the double ALL the time. They are great stroller riders and great walkers. Truth be told, we only recently started leaving the stroller at home. We're going to WDW in a month and we're bringing the double (my kids are now 4 and 2.5). My daughter has and still is a pretty happy stroller rider. Plus, I find that it is nice to have the empty seat to carry stuff (jackets, bags etc) if we started off walking in the stroller but then got out and walked part of the time. I think it is easier than bringing two strollers (which requires two adults to push at all times-- instead of one pushing and one chasing if you end up with a walker for part of the trip). We also do a fair amount of afternoon walks when both kids sleep in the stroller to get my older DD to get a short nap in (she won't nap at home but often falls asleep in car or in stroller). As a mom, I can tell you it helps my sanity to get in that exercise while they are napping and makes the rest of the afternoon a lot more manageable if both kids get their nap in. We got a BJCCD when it was being discontinued. I'd say try and get a used side by side. To be honest, I'm expecting number 3 and I'm dreading the fact that we'll only have 2 stroller spots with the double.
Good luck!