I, too was undecided between the Rally Twin and the Twin Traveller. I ended up buying a TT without having a chance to look at the RT. Shortly after we bought the TT, I saw a RT at Babies R Us so I was able to compare the two. Both look similar and fold the same, but here are my thoughts for whatever it's worth:
1. The RT feels a bit lighter that the TT, but not by much.
2. The wheels on the RT are smaller.
3. The RT's seats are narrower than the RT (probably a 1/2 inch difference for each seat, which made me glad that I have the TT since my kids a little older and bigger now).
4. The RT and the TT's recline system are different. The RT uses a buckle and strap to adjust the seat to any position, much like what my jogging stroller has. The TT uses a recline similar to the other strollers I have...a "ratchet" system (for lack of a better description) that adjusts to five positions, I believe.
5. You get more "stuff" that comes standard w/ the TT: Leg extenders on the seats, the head support, boots for each seat, bottle holder.
Both strollers steer well and are sturdy, so either one would do fine in that area. If you're worried about the color, go with what you like.
HTH!
Good luck with your decision.