I may be able to respond to this easily from a low level standpoint, as I am just returning to WoW after a year + hiatus. I am currently working on a Druid, Shaman, and Rogue... but I have more or less left the rogue to die in a tavern drowning his sorrows.
The Druid is incredibly fast of all the classes I have played, and prior to returning I had high 60's or 70's of every class except mage and shaman, as I played on the Alliance side and did not reroll when TBC came out.
Now I am playing horde with some RL friends and so we are learning all the quests and so forth. Since you are not asking about druids, I will leave them alone. However, with regards to the OP, Shaman over Rogue, I have found that up through the high 20's they were relatively equal and now I am leaving the rogue behind as I begin to hit my stride in the 30's. Initially you will be doing about the same things in PvE with regards to soloing in terms of breaks... The Shaman can solo just as well as the rogue if not better as you can survive adds whereas the rogue often has to vanish. This is offset at level 30 with Blade Flurry to make it easy for the rogue to deal with adds pretty effectively. For speed, it has been mentioned often, ease of leveling is the same and can be better for the rogue, but most people play the rogue so they use situational abilities, such as stealth and so forth, which in truth slow down your overall speed of play.
For grouping, at low levels it is fairly easy either way to get in a group for instance runs. Shamans have utility and Rogues have better pure dps at lower levels. However, with the new streamlining of leveling, I have found that you can run every instance up through high 30's with 4 toons assuming a tank, healer, and 2 dps... without needing a 5th. Before recent changes, if I remember correctly, you rarely ran an instance without a 5th unless you were all at a level where you were consistently fighting greens.
So to answer your question, Shaman seems faster to me period even from low levels because you do little stopping other than to drink / eat, the same as a rogue would. The difference is that when I played my rogue I played them as a rogue, which meant positioning for a cheap shot or ambush and then rinse and repeat. So I would say go with a Shaman as you already stated you liked Shamans and if you are posting in the Shaman boards, then I feel you may be looking to be swayed.