Is he right or is he just being smug?
I think there's some truth in what Williams, Gatland, O'Gara and Marshall all say. The top Magners League teams (Leinster, Munster and Ospreys) are some distance better than the best in the Premiership in my opinion. Others like Cardiff Blues and the Scottish regions are very competitive.
I agree that Munster and Leinster are currently better than the top guiness premier sides, but to me, and you may disagree, the reasons for this are quite straight forward; with magners league being the top competition for Irish teams to compete in, there are very few top Irish club sides( Ulster to me are average, connaught below average, and leinster and munster are obviously very good), but the fact is if you're a top Irish international and you want to play in Ireland for a top Irish club side, you only have a few choices. To me this is why the Ireland national team is essentially Leinster + munster. Similarly with the Welsh teams, I think Cardiff Blues are good and Ospreys great but again they are essentially the welsh national side .
In the Guiness premiership internationals are more evenly spread out through the league, and I do think this has made it a more competitve league, but perhaps thats why there are fewer exceptional teams.