Braising in the Oven
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The difference is negligible. I personally lean to the 325 on the theory I'd render more fat but that's just an instinctual hunch. I'd have to try it both ways and see if one was less fatty than the other to be sure. But the difference would be small if any.
If you like your pork belly fattier than I do, then my hunch might not be useful to you.
Both should be done completely at the end. And both will likely be just as good as the other.
I'd guess that the recipe writer had slightly different goals. One wanted simplicity so specified a single temp for a single duration. The other found some subtle reason why more involvement would be good so specified different temps and times.
So suggest you pick one and give it a go.
I like my pork belly quite fatty so I'd probably go 275 for 3 hours and then crisp the skin. :)