Clark
I think Clark failing to utilize the gap in German lines following Monte Cassino allowing Germans to escape wasn't a great move on his part....
"After the negotiations, Clark was promoted to lieutenant general on November 11, 1942. When the United States created its first field army overseas, the U.S. Fifth Army, Clark was made its commanding general and given the task of training units for the invasion of Italy (Operation Avalanche) in September 1943. According to Montgomery, Clark was subsequently criticized by British historians and critics, for the near-failure of the landings at Salerno, as a result of perceived poor planning.[5]"
Mark Wayne Clark - Wikipedia, the free encyclopedia
Clark didn't seem to have a lot of people on his side.
Kesselring
Kesselring had a near perffect coordination between the three services under his command, he evacuated 40,000 men, but also 96,605 vehicles, 94 guns, 47 tanks, 1,100 tons of ammunition, 970 tons of fuel, and 15,000 tons of stores.
Kesselring was a bit over optimistic as stated before.
Which one was a better tactician?
I am going to say that Kesselring was a better tactician. He was over optimistic. I think Kesselring was a straight forward general. He wanted this done now and didn't want to wait. Clark on the other hand ignored orders during the Battle of the Winter Line, failing to use gaps in German lines was also a bad thing (allowed Germans to escape and reinforce the Gothic Line) and the almost failed landing of Salerno are nothing to be proud of.