the lost levels = super mario 2 in japan btw, anyway both games are very much the same the levels look the same, and the gameplay feels the same, so its hard to say which is the best, But i would go for the lost levels as it sounds cooler
Its as simple as: Do you want the classic "easy" version, where you know exactly where everything is? Or do you want a hard ass version where everything is in a new spot......even going in the bonus rooms in the pipes, you surprise yourself by finding power ups you didnt know were there last time you went in the room.
I say, get them both. No really, if you want retro, get old. If you want new challenge, get lost levels.