You are basically conceding Murray is better on hard courts so that already means he has a better shot. Hard courts is the best surface of both and where their most likely slams will come. Yeah Djokovic is better on clay but neither is going to win the French with Nadal there, and even if Nadal goes out there is Federer and Del Potro to deal with both who would both beat Djokovic in good form right now.
Your statement of anyone in the top 30 being able to beat Murray on hard courts is really blind hating. The lowest ranked player to beat him on hard courts in a long time is Cilic, and Cilic is a fast rising up and comer who will be top 10 very soon.
Comparing Djokovic and Murray on HC slams
1. In AO and US from 08 to 09, Djokovic only lost to Federer and Roddick (and this was partially because of the heat), and in his matches with Federer, he gave a fairly good fight
2. In those same tournaments (AO08-USO09), Murray lost to Tsonga , Verdasco, and Cilic, and his match with Fed in US 08 was a complete demolition
The only HC slam in which Murray did better than Djokovic was USO 08, and in that tournament, both Djokovic and Murray lost to Federer, except Djokovic pushed Fed to 4 sets while Murray was anihilated in straights
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On topic, Del Potro will definitely have more slams than Djokovic. His talent is probably better, his fitness is no worse than DJokovic based on what I've seen this year, and he is more concentrated on his tennis than DJokovic. Djokovic may well end up with only 1 slam sadly.