From my research the enzyme level used to be higher in 2 row but through years of selective breeding there is really not much difference in enzyme level between 2 and 6 row now. Its also(unless i am remember incorrectly and getting this bassackwards) my understanding that 2 row is commonly used throughout the world outside of North America, but 6 row is more commonly used in North America. Since my main interest is bourbon and my understanding is bourbon makers use 6 row, i use 6 row. I guess if i was trying to make an irish i would use 2 row.

In all honesty i have used 2 row instead of 6 when 6 row wasn't available, and really didn't notice any difference in mashing or flavour. I don't think you can go wrong with either. I've also added small amounts of the specialty malts and found they can add interesting flavours as well.