I think either player gives it all they can, I cant fault either for work rate. It might be that Parker has more stamina, and therefore covers more ground (or vice versa) - but thats nothing to do with workrate rather physical limits.
Number of tackles etc is not a stat to see in isolation, different opposition, systems, home/away... so many things to effect that.
I think Parker is much more capable of playing at a higher level game after game than Sandro. I think Parker is much better at playing a restrained, disciplined way as well.
Sandros performances veer from the sublime to the ridiculous, and he does have a tendancy for a 'rush of blood to the head'.
Neither can be proven, it is of course subjective...