The .012" lines will still be cleaner in the air than any dacron and spiderwire and these are also what is required for competition. In mouse racing, .012" 42' lines are required. While .008" may be preffered for performance and sport flying they are fragile and if going with braided steel lines, the beginner would be better off with the larger .012" I believe.
This is certainly something to worry about later though and I would concentrate on getting in the air. Availability of these items will be a big factor. I have always bought my lines in bulk (being a combat flyer) so I have plenty laying around.
Also remember that when you begin to do aerobatics, the longer lines actually give you more room to manuever. I would not suggest going under 35'. You lap times could be so short that you could get dizzy.