I believe the difference is resistance to fatigue under specific circumstances. The 35's are meant for engines that are heavily modified such as port work, decking or trenching, raised rev limits, and complete timing curve changes. Essentially, if you plan on doing at a minimum, a sport porting to your exhaust ports and a higher than stock compression head, the 35's will be a better choice because they are made to withstand the new and increased crankcase pressures as a result of the mod work. Reed selection is relative to the level of modifications done to the engine. When more airflow demand is placed on the engine, more stringent measures for sealing that engine come into play. The stock stainless reeds are considered the best of both worlds as far as high end modification requirements and reed cage sealing but I have seen them literally self destruct breaking off chunks. That could really ruin a good day of riding pretty quickly. It can happen with new ones too, it just depends on the quality of the metal for that production run. One bad stainless reed could write off a brand new or newly rebuilt engine in seconds. Here is the product description for the PRO-35 reeds from Boyesen...
"Boyesen Pro Series Reeds combine breakthrough carbon technology with all the advantages of Boyesen's dual-stage Power Reed. The High Performance line is intended for High Performance modifications and/or racing applications."
http://www.boyesen.com/high-performance-pro-series-reeds-3.html
