I don't think it's a gimme. Both can become free agents the same year. Maybe that's about the time Donaldson starts to decline while Machado is peaking. If I'm the low budge A's, I'd want nothing to do with Machado and FA(assuming he doesn't shitcan his career by acting like a 9 y/o). So, for me, I'd figure out who was going to be the best player 2015-2018. That's why I don't think it's a gimme.
The biggest factor, IMO, is who I think Donaldson really is. Players who "find themselves" in their mid 20s concern me. I'd have to offset that with Machado's behavior.
FWIW, I think Donaldson has probably had his best year but I also think Machado is going to be an off-field/clubhouse problem. Maybe he grows up. It's just not cut and dry like you implied.