I believe there were fewer test market cities for the 35 centers as well as a shorter duration. Also, they are much tougher to distinguish from the regular versions compared to the 30 centers which often had a gaudy starburst surrounding the price. Still there are some 30 centers that are as tough if not tougher than 35 centers to find.
I think Both 30 & 35 were supposed to have been released in 6 test Markets and both ran for 5 months through all of Marvel's regular lines. I used to believe that the numbers of existing copies for both should be very close but there not, I think it was JC that said long ago "that the numbers are not even close on 30's & 35's" but I thought I knew better back then, boy I know now I was very wrong. And even the tough ones do not compare, there are probably at least 5 to 1 or more on the 30 cent Westerns and Fury's compared to the 35 cent ones "I guess the print runs dropped off a little on those by 1977 so maybe that has a little to do with it".