[SixOfSpades]

First off, there's a difference between Rogue and Thief: The "Rogue" classification denotes all Bards as well as all Thieves. Next in importance is the fact that if you've got Throne of Bhaal installed, you're not going to want to play a pureclassed Thief: By the time you're 1/3rd of the way into the expansion, even an Assassin is going to be above 100% in all Thief skills, so essentially you're going to spend quite a lot of the game not getting any better.

But now to your real question: Thief vs. Assassin. The Assassin does get better at Backstabbing, of course, but the plain Thief is no slouchie either. And the Assassin's x6 and x7 multipliers don't even kick in until you reach the appropriate levels, so for the first half of SoA, the two Thieves will have equal Backstabbing power. This means that in early/mid SoA, the Assassin's Poison is his only advantage, and happily it's quite a potent one. Combine it with weapons with a high number of Attacks per Round (such as Darts) or that are poisoned already (such as Bolts of Biting) and most enemies will be literally unable to move due to taking so many hits from the poison damage. Then again, getting only 15 Thieving points per level is a big turnoff.

Summary: You won't want to be a pure Thief, so consider what class you'll want to combine with. You could Dual-class (Assassin->Fighter is a popular combo) or Multiclass (Mage/Thieves are also favored), whichever you prefer. Also, consider your party: If you want to be an Assassin, you'll probably want to bring a backup Thief (such as Imoen or Nalia) along to help support the party while you bone up on your Backstabbing skills. A straight Thief (or Swashbuckler), however, can provide good support right from the get-go.