I'd probably try to tackle a shooter in preference to playing dead, but not in preference to any other viable response. Since I'm usually both armed with my carry gun and carrying pepper spray and a tactical bright flashlight, I'd usually have better options than pretending I'm a football linesman. About all I can say in favor of me tackling a shooter is that I'm big enough to block his shots at other people at least briefly. (wry grin) The problem with that is that I'd be drawing fire, and not likely to live through it if the shooter is competent. All things considered, I'd rather live.
However, if living isn't an option, I think that I'd choose to go out fighting.