Here in the states they did away with the DP Magnum 660, and the 660 here only has 7 HP, so the 661 is a bump in HP. In Canada and Oz, they still have the older 'real' Magnum DP muffler covers, but anyone can order one and pop them on here for the added power. The 661 has a lot of new features, including M-tronic, smooth spring loaded AV, new/better oil pump, and better AF. People that ran them liked them.
As for the best start, the original 661s were withdrawn in the US, UK and EU, and production was stopped on them. Not the best rollout of an enterprise product line. Word is that the cylinders had some serious issues and were failing in a high number of saws so they are redesigning the jugs. They will be out again in the Aug/September timeframe here (according to Stihl USA).