I detail 3 convertibles every month in my part time, mobile detailing gig. Very different conditions. The brush I use depends on the convertible top.
1- 2010 Jeep Wrangler- top is sun damaged, torn, patched, etc. - for that I use a soft brush
http://www.chemicalguys.com/Body_Whe..._p/acc_g09.htm
2- 2013 Camaro 2LT- the owner bought it used and it needed help at first, now garage kept - I used a natural hair brush like this one
http://www.chemicalguys.com/Converti..._p/acc_s94.htm
3- 1994 Porche 911 13,000 miles garage kept. Small boars hair brush , very gently, once, then just spot cleaning with softest rags.
I like Chemical Guys and 303 Convertible cleaners and protectants. Theyre not the cheapest but my customers want the best.