As previously mentioned there is no real winner as their capabilities are different. But having served on the ground in Helmand I found the Apache to be more effective in Strike but the Harrier for ISTAR.
The Apache was very handy with its load of 30mm, Rocket and Hellfire. 30mm being devastatingly accurate and plenty of it, rockets would bring down tree lines nicely and the Hellfire was great for accurate punch. The problem we had down South was that it couldn't hang about too long compared with Harrier.
The Harrier could sit nice and high to watch with its pod and could remain on station for a decent time which was probably it's most handy trait for us. It could provide serious impact with larger bangs than Apache, great for simultaneous strike. And great for clearing crowds etc with 200' show of force.
At the end of the day the Taliban was a fan of neither and they both did great jobs!
LZ