There are definitely differences between GAOP and ACOP positions. GA has alot more variety of work tasks but can be overwhelming most times. Imagine handling 10+ pilot deviations along with airshows, inspecting 135, 141, DPE's, Check Pilots, 91 RVSM, Noise Complaints, Initial CFI practicals, etc. The work keeps piling up, and you never catch up. The ACOP deals with one airline, and thats all. From a workload standpoint, GAOP works their tails off for the meager salary they make IMHO. That is why GAOP becomes (for most) a "burnout" career move. Once you realize you can make the same $$ for half the stress and workload, you won't want to go back to GAOP. I've done both and much prefer ACOP work.
As far as promotions go, alot depends on your initiative and motivation to get your OJT done. There is ALOT of OJT requirements, and get well familiar with FAA Order 8900.1. If you printed this order out, it would be Thousands of pages. Seek all the OJT opportunities you can, and do not " wait" for OJT opportunities to come to you. Once your FLM and MGR see you are motivated and well on your way with OJT, then the GS-13 should come along appx. At the 1 year mark. 14's are more difficult to come by.