I don't think you will go wrong with either action. That being said the GMA action is well known in the custom gunmaker circle. Hein is just getting their business started. The other reason is the delivery time. You state that a GMA can be had in 5 months....Hein will be 12 months or more. Now if you would like to have some special features added to the action like special scope mounts or sight options...then Hein may be the best place to go. Once again...from what I know (and that is not very much) both of these guys build great products...you will be very happy with any one of them.