What is the difference between Swiss Meringue Buttercream & Italian Meringue buttercream & is one better than the other?

Basically it's a matter of how long you cook the syrup. And no, one is not better then the other, it is just a matter of preference. I find they both taste wonderful, but I prefer to make IMBC.

Swiss is made by mixing sugar and egg whites over a water bath until the crystals are dissolved then whipping it until you have a meringue with medium peeks then adding in softened butter. Italian is made by whipping egg whites till foamy, add a bit of sugar in like you were making a French meringue (so it takes the boiled sugar better) then pouring in softball sugar and whipping until medium peeks and again adding in butter.

As for which is better, it's personal preference. I prefer Swiss, it's more stable and silkier. Italian is airier with higher volume. Italian by no means is unstable, just less stable than Swiss.