Slot layout looks to be the main thing.
Possibly a difference in overclocking ability. But the P43 is no less reliable or anything like that. P45 can take 16GB of RAM, P43 seems to be a maximum of 8GB.
Will you be overclocking?
Why, do you like pain?Ive got a P35 board but im missing my nvidia SLI board
The Asus P5Q range are right in your price bracket and are all P45 chipsets. I went for the cheapest and it's a great overclocker. If you have £80 though I'd go for the cheapest that has 2x PCI-E slots just in case you want to go xfire sometime in the future.