The Theif, like actual Rogues, is a great damage-dealer and hits quite well (the rogue hits even better, with all it's non-AC-targeting powers), so yeah, it's a good striker and probably delivers higher peak damage than an indifferently optimized Slayer.
The Slayer is really quite tough, though, with Defender hps and surges, and heavy armor if he doesn't go in too heavily for DEX - that should count for something, too.
Most Rogues I've seen run into trouble not with getting CA so much as staying alive through a normal adventuring day. Maybe the Theif can establish CA at range a lot more easily, and thus stay out of the trouble most Rogues get into?