Longboarding is sick. 36 or 42 inch would be straight. Depending on what type of longboarding you want to do! Skateboarding is also pretty cool. I used to skate but I broke too many boards and I always managed to fucking injure myself lol.
If it was me I would look at Sector Nine as they are good boards and very lastly depending on how you treat it. I had my Sector Nine for almost 5 years until somebody stole it.
Skateboarding for sure... Longboarding is gay as fuck. I see people walking up and down hills all day riding down the same hill. Also if you want to " Cruise " just go out and buy a cruiser. Its lighter and probably way cheaper.