If the jumping attack connects the force would be stronger because it's your force plus momentum, plus your bodyweight, plus the power from your strike that multiplies everything. If you are stationary it's only the momentum from them moving forward towards you that you can take advantage of, your stationary strike won't have the extra weight and momentum behind it that a proper jumping attack would.
Plus it's really hard to time and counter a real jumping attack that's done right. It's not worth the risk staying stationary trying to counter it. It's like staying still and trying to punch a defensive lineman running at your full force trying to tackle you. Even if you hit, the force would move you back easily. Especially if the jumping attack is a kick, there is no punch counter to that. That people that have got hit in the past have been due to sloppy superman punches, or jumping knees, that had bad timing due to being fatigued. Look at Yair, Zabit, Tenshin, no one is countering their jumping attacks - there is a reason why they don't.
