Longboard skateboard questions, help?

I want to get my boyfriend and I longboards for our one year, he's been looking into it and thinks it would be fun. He's skateboarded before but not longboard. I've done neither, haha. So, what's the difference with the wheels? I've noticed a lot are either 70mm or 76mm, which would you suggest for beginners? And also is longboarding more difficult than regular skateboarding? And if you have a good online skateshop that carries longboards that would be helpful, and any other suggestions or tips would be cool. Thanks:)