Yeah exactly. I'm not a big fan of the "overall contribution to their team's success in international cricket over the course of their careers" argument. Because, in theory, you could have a batsmen who averages 50 consistently over their, say, 10 year career and another one who averages 70 in the first 5 years and 30 in the last 5. For the sake of the argument, both could be said to have the same overall contribution to their team's success, but only the latter would have really dominated attacks at one stage. Also, by averaging 70 for 5 years they would have reached a skill level not even touched upon by the other batsman. That being said, though, the other batsmen would obviously have better longevity to compensate for that.
However, if I had to pick one, it would be the player who once averaged 70, simply because it shows they once were capable of playing at such a high level, and I think the skills required to do so are harder to come by than attributes required for longevity.