I debated a lot between these two strollers. I ended up getting the Twin Traveller. I'm pretty happy with it. The biggest difference that I saw betweent the two was the shade canopies. The 2005 Twin Traveller had a much more generous canopy than the Rally Twin. That's what sold it for me. Now, granted, I did not see the Rally Twin in person, so I am basing this on online photos of the Rally Twin (and canopies on Maclaren singles in the store). Shade is a huge deal for me, so I went with the one with the larger canopies. My girls were 3 and 1 1/2 when I got mine, so the head rests and leg extenders weren't really used.
What I like about the Twin Traveller:
good shade canopies
folds compactly
has reclining seats
55 pound weight limit per side
separate baskets under each seat
well made and sturdy
easy to push and steer
seat covers can be washed
What I don't like:
hard to steer one-handed
doesn't stand up on own when folded
can be tricky to unfold
NO cup holders (the single one broke very quickly)
not as light weight as others
It's not a perfect double, but I did a lot of research and figured that since it had the highest weight limit per side, it would last me the longest.