Maybe just my opinion here but I am always looking at down the road. If you go for larger 70/72 size it will work just fine and the engine will be able to power most other 40 size planes with good authority. 4 strokes are twice the price of 2 strokes so why not get one that will be able to fly more planes? Its about $30.00 more now vs. the price of another new engine.
[8D]