"A 3W-24 is a whole lot heavier than the RCS 140."

The difference in wt is 10.4 ozs with the 3w-24 being heavier. With a 10 lb plane, a 24 powered plane would be 6.5% heavier and the engine produces 11% more power. I would choose the 3W-24 on these numbers alone. In addition, the RCS-140 is fragile with poor mounting flanges prone to easy cracking and breaking (personal experience). The extra wt of the 3W comes from more robust cooling fins and larger engine components like crank, bearings and engine mounting flange. If you add the weight of the required engine mount for the RCS-140, the "perceived weight penality" of the 3W almost disappears leaving the flier with 11% more power!!

Sometimes it is better to look at the whole weight/power picture than just weight alone.

Just my opinion

Elson