Based on what? I don't want to tear down Frank, but his prime, as mentioned was 91-97. 7 seasons. Mays had 13 straight seasons without having an OPS lower than .924. He also never played fewer than 151 games in that stretch. 4 times in that stretch he hit more homers than Frank's career high. He also hit double digit triples 5 times in that stretch. Stole double digit bases 9 times (do steals not count, it is offensive but not hitting. Ok maybe they shouldn't count). He knocked in triple digits 10 times in that span. He didn't BB like Frank did, but Frank had over 350 TB once in his career, Mays did 7 times.
There can be a very strong case made for either guy. To try to claim it definitively, is silly.