Oh that makes sense. So the first option is more “it is difficult for us to determine who is faster” whereas the second be: “we know who is faster, but it’s hard for us to say it for a different reason”
Peterdg is right about the possible second meaning of the second one.
'Het valt moeilijk te zeggen wie er sneller is '
is perhaps the more usual way to say this and also my preference
Personally I'm shunning 'om' when it can be said without. So I don't like the second sentence.