I have many dozens of CoC books from 1st edition to 7th. I can use all of them in CoC 7th Edition with only minor on-the-fly modifications. CoC 6th edition is also a solid ruleset. If you want to play with the latest CoC version (7th) go ahead and play with that. If you're happy playing in 6th edition then play with that - you'll have great fun whichever one you choose. :)