With the Saito 150 I had to use a brass "heavy spinner hub" and place the battery all the way forward, up against the firewall under the fuel tank, to get it to balance. Didn't have to add any weight though.

If you're more comfortable w/ 2-strokes, by all means get the OS. Mount it inverted and use an inverted pitts-style muffler for a clean set-up. After experimenting w/ several props, I've found the APC 18X8 gives the best overall performance on the OS 160.

For a full review I wrote a while back on the Sig clipped wing Cub, go to: http://webpages.charter.net/rcfu/Kit.../SigCWCub.html