I've always seen these arguments and wondered. I know that my weight is considerably more than my bike (200lbs with a bike in the mid 30s), so me losing 15 pounds seems like it would be way more of a contributor than dropping a few pounds off my bike, but I don't think so. I say this because the bike is an extension of the body; an outside tool.

By being like this I think the effect of the weight is multiplied, especially noticeable in technical parts of trails. Think of it like golf or baseball. Add a pound to the club or bat and it'll be a huge difference (the physics of swinging something makes the effect more dramatic than riding, but same theory).

Like when people who are heavier say 'try running with a 50lb backpack on,' they're incorrect. It's not the same. Their bodies have the weight incorporated and distributed in a more 'natural' way.

With the heavy rider with light bike versus lighter guy with a heavier bike, I think the advantage is to the heavy rider. There is less of an effect from the weight of the bike, and also when standing up he has 15lbs extra to drive the pedal down with.

At least that's how things work in my own little world... Strippers are also very nice people in said world.