Two graphs: one of the Democrats and the delegates under a winner-take-all and proportional system and one of Republicans with the same.
On the Democratic side, which uses the proportional system, a winner-take-all format would actually make the contest more competitive whereas on the Republican side, which uses many winner-take-all contest, the proportional system would make the system more competitive.
The Democrats:
The Republicans:
It seems that no one method is better, they're just different. Having said that, looking at the Republicans, it seems the winner-take-all format really screwed Romney.
On the Democratic side, which uses the proportional system, a winner-take-all format would actually make the contest more competitive whereas on the Republican side, which uses many winner-take-all contest, the proportional system would make the system more competitive.
The Democrats:

The Republicans:

It seems that no one method is better, they're just different. Having said that, looking at the Republicans, it seems the winner-take-all format really screwed Romney.