No offence to the OP, it's a fair question, but I'm going to say neither. Routh, forget about him, not even in the same league as Reeve or Cavill, there's a reason he's not a household name after playing Superman
Now with the tone of MOS, I don't think it would have suited Reeve's tremendous presence and boyish charm. I'm not saying he's not great, and more importantly he
was a great Superman, but the particular tone and approach taken in MOS would not have suited him -just not the right man for the job.
If MOS had had a much lighter tone and a bit more humour, then Reeve would have nailed it.
So, there are probably quite a few guys today who could have pulled it off, but Cavill was the guy they chose. I think they chose right. The second best performance as Superman, IMO (and that includes Tom Welling and Dean Cain), he didn't have Reeve's warmth or charm, but you certainly feel he was a sincere, earnest Clark Kent, burdened by his great secret, and of course, we know he cares about his mom.
Also, we haven't seen Cavill as Clark Kent journalist -although when we do, the dynamic with Lois will be totally different (and a lot of Reeve's key Clark moments involved Lois).
IMO they picked Routh back in 2006, because he looked like Christopher Reeve.
They picked Cavill, because he looks like Superman. Reeve changed the way Superman looked, but when Cavill walks out wearing the suit in silhouette the first time, it's like Superman walked out of the comics (minus the trunks of course).
So, if I had to choose between Reeve or Routh, no contest, it's Reeve, but even then he'd be the wrong guy for MOS.