Statistically Lendl beats Kuerten in pretty every category on clay. He had a better record RG having reached 2 more finals, he won 28 clay court titles compared to Kuerten's 14 (even if you want to ignore Lendl's green clay titles he still won 16 titles on red clay), and he won more 'big' clay court titles winning Monte-Carlo, Rome and Hamburg twice apiece while Kuerten 'only' won Rome and Hamburg once each.
Also Lendl had clay court rivalries with Wilander and Vilas who I would say were better players on the surface than anyone Kuerten ever faced. Beating Vilas twice in big clay court matches in Buenos Aires was particularly impressive.
I do think that Kuerten was far more enjoyable to watch on clay than Lendl though with his artistry, compared to Lendl who featured in so many moonball-fests on the surface (the 1987 RG final was horrific).
Still he was far less reliable and more vulnerable on clay than Lendl ever was I felt. In his prime for instance he got his *** kicked by Hewitt in his home city in the 2001 Davis Cup.
Lendl won pretty much all of the biggest tournaments available to him on red and green clay (he won the biggest green clay event of his era, Forest Hills, 4 times)