I opted for the WM Ultimate 40s. I started with an OS .50sx and a 12x4 APC but recently switched to a 13x4 APC. I like the 13 far better. The speed this little bipe wants to fly at is amazing. I have my battery in the fuse behind the lower wing to achive balance and thats a 1700 MAH 6V Nmh pack. Anyway the WM 40s has put up with a lot of abuse (aka hard landings) with no damage other than having to re bend my landing gear. It would have ripped the gear block out of most ARFs. Anyway after I figured out that I set my Expo rates the wrong way (made it more sensitive to inputs) I fixed that and this bipe is a pleasure to fly and always a croud pleaser. Oh and the 50sx with either prop gives this bird rocket like virtical performance.