Without injuries, I will give Guga the edge.
Guga's game matured at 2000, especially on surface other than clay. After winning TMC 2000, he's confident he can win on faster surface and even though he was playing injured during US hard court season in 2001, he managed to win TMS Cincy, lost to Agassi SF in LA, lost to Roddick in TMC Canada, retired in the final vs Rafter in Indianapolis and QF to Kafelnikov in USO. All these are quality players on hard court.
Without his hip injuries, he can definitely challenge any top 10 hard court players and continues to reign on clay courts a few more years. Guga's serve is very very good and he can hit with power and precision from both wings.
I agree with you, Lee, that without injuries Guga could quite easily have run away with the lead. (Going by actual results, however, I have to lean towards Hewitt).
Anyway, if there's any doubt that Guga could play on hard court, here's his 2001 Cincinnati draw!:
1st Round Andy Roddick (USA) 24 7-6(3) 6-1
2nd Round Tommy Haas (Germany) 22 7-6(4) 7-6(10)
Round of 16 Goran Ivanisevic (Croatia) 11 6-2 6-1
QF Yevgeny Kafelnikov (Russia) 10 6-4 3-6 6-4
SF Tim Henman (UK) 8 6-2 1-6 7-6(4)
Final Patrick Rafter (Australia) 4 6-1 6-3