From what I understand, and I've been wrong before, the 3303 was developed specifically for the ACC and was produced before the 3313. Omega may put the 3313 in future production of the ACC, the primary difference of course being the co-axial escapement. That would make it a much more expensive watch, though. I like the fact that the ACC has an interesting and durable movement and is sold at a reasonable price.
Here's an interesting excerpt from a post by Marcus Hanke on a UN forum concerning developement of the 3303:
"...the Omega movement [3303] of the Seamaster [AC Chrono]: Being a designated premium brand within the Swatch Group, it was clear that Omega should go beyond its hitherto used ETA/Valjoux movement bases. The Piguet 118x, which was produced within the group, was an obvious choice. However, Omega always had a reputation of producing sportive watches for everyday use, and the Piguet movements were already infamous for being fine, but delicate. Therefore, Omega engineers teamed up with Piguet specialists, and created a new chronograph movement, based on the 118x's general layout. However, the new 'engine' was designed larger, in order to better withstand shocks. The subdial layout was changed into the more traditional configuration of the small second at 9, 30 minutes-counter at 3, and the 12 hours-counter at 6. Finally, it was designed from the outset to incorporate George Daniel's co-axial escapement, which is exclusively produced by Omega. While the actual movement production is made by Piguet, it is no doubt that the main development effort must be credited to Omega, and I think it is only just that Omega claims this movement as its own."
So, from what I read here, the 3303 was developed first, and "designed from the outset to incorporate" the co-axial escapement [the 3303 with the co-axial escapement is the 3313, am I correct?].
