Been to both but cant decide which I prefer. More livelier in G.C., Fuerteventura is quiet and can be windy, most bars close around midnight and everywhere looks deserted, just depends what sort of holiday you are looking for.
Parts of G.C. are not very senic so choose were you stay if that is a concern. Weather wise G.C. is probably a better bet.
Just do a bit of research and decide which suits.

I love all the Canary Islands, and you'll probably get a better idea of what would suit you if you ask a few questions on the Gran Canaria and Fuerteventura forums
I would say in July you're going to get fantastic weather whichever one you choose. If you want nightclubs etc, then you need to look at Gran Canaria. If you just want nice bars and restaurants then both islands have good resorts.

Many thanks to you both having weighed up all my options i have chosen gran canaria, just can't wait to go now!