If you're a CFI, sitting around 1200 hours, with a job offer from AMF...just build 300 more hours and go to a regional.
I really enjoyed my time at AMF (mainly because of the good people and fun base) but that's all it was, flight time. My final goal is 121/corporate, and I will get there quicker coming from a regional. I left at my first regional offer and have no regrets. AMF management will tell you otherwise..."get 1000 TPIC here...it will make you a better pilot...etc" - but they don't tell you:
1) Majors will hire you with 4000 121 SIC and no PIC...the same can't be said for guys with 1000 135 TPIC and no 121 experience. They are destined to go to the regionals to build that time. So since you will be going to a regional anyways to get the 121 experience, it makes sense to go earlier and get better seniority.
2) Logging lots of single pilot IFR doesn't make you a better aviator, it makes you a better single-pilot IFR operator. Flying a multi-crew jet with lots of glass and automation isn't easier, it's different, and requires a different skill set.