I prefer 9 footers because outside of league, that's where most of the action is happening. But I think if you want your game to be respected across all area's, you need to be just as comfy on a 9 foot table as on a barbox. I feel the same about playing with a house cue vs personal cue as well. Also think knowing more then just 1 or 2 games on both tables is important.
One reason I like barboxes alot, and I don't think it was mentioned yet, is the variety of players you come up against. When playing on 9 footers, you are usually going to play some serious, good players. They came to the pool hall just like you wanting to learn, win and try to improve their action. In the bars, you never know who you will play and that is half the fun. Even if the equipment is crappy, sometimes your opponent or the crowd makes the games fun. A wise player will know when rails are bad and how to compensate. He will also know how to play clusters, by looking at them like playing straight pool or 1 pocket. He will know how to make the cue ball dance even though there is not as much room on a bar table. Having the same number of rails but in a closer proximity can lead to some amazing shots/sets where you don't have to work the CB as hard. And when you do match up against a strong player, he will be playing on the same crappy equipment, it's just a matter of who can adjust faster.
Bottomline, I have seen some great and horrible tables in both sizes, but people still play on them and I think there are lessons to learn doing so.
ez