The Democratic primaries all allocate delegates proportionally; a fair proportion of the Republican delegates are winner-take-all.
The conventional wisdom seems to be that that means that "all other things being equal", the Republicans will choose a nominee sooner than the Democrats, because a Republican candidate can build up a large lead in delegates more easily than a Democratic candidate.
But the arithmetic works both ways. A Republican candidate can build up a larger lead in delegates... but a lead in delegates of, say, 10% of the total number of delegates is a lot less safe for a Republican. I suspect if one analyzed it properly -- asking questions about how the probability of a given candidate winning changes during the primary season -- the two systems wouldn't seem all that different.
A system like the Republican one in which some states are winner-take-all and others are proportional, though, just seems too asymmetrical to be stable. Quite a bit probably depends on whether the election is close... so in the end it probably turns into a question where candidates and their people try to argue for one method or another on ideological grounds when they're really just trying to calculate what makes them most likely to win. I suppose you can't blame them for trying.