The import from classic to 7.0 will import the most data. classic to 6.5.5 will only import forums, categories, posts, users and PMs. The 7.0 importer will grab much more including polls, moderators, etc.
Since we're just about ready to release the final of 7.0 I'd probably spend your time working on 7. Any time you spend on getting 6.5.5 to look like your old site will be a bit wasted as 7.0 has a whole new style system so you'd need to do all that work over again when you switched.
