Originally Posted by IS500Z
Absolutely stick with one of the 'pro' models if you can afford it. If not then a MS280 would be a good choice. For an 18-inch bar you would want around a 60-cc motor. The MS290 is Stihl's best selling saw, but it is also the least expensive 50-cc saw they sell which is why. You pay for that low cost with the worst power to weight ratio in their line.
That line is not true at all. I've been cutting for 30 years, and I've used a lot of saws over that time. My smallest is my 45 cc Jonsered 2145, and it pulls my 18" bar with no problem even in oak with the bar buried all the way. In softer woods like maple or pine, it does it with ease. I ran an 032 Stihl (51cc) for 20 years with a 20" bar, and it was never lacking power either. An MS 250 is alll the OP needs for his intended use if he is wanting to stay with Stihl.