You could get a total output of 300W from that amp regardless of whether you get the 4ohm or 8ohm subs. If you get 4ohm subs, you could wire them in stereo and get 150W to each sub. If you get 8ohm subs, you can bridge the amp to split 300W between the two. There should be no difference in power output, so I don't understand why he'd say you would get more power one way or another.

Personally, I'd rather get the 4ohm subs than the 8ohm - it'll be more convenient if you ever change that amp.