I say get the thunder tiger 60 ARF trainer, as it's got a bigger engine in it (and it's got a bigger span too), plus you can learn to fly using a 61 size engine trainer). If you where getting the Sig LT-40 in kit form then I would put in a 61 size engine, as the ARF one has only has a 40 in it. The thunder tiger 60 ARF trainer, or the Sig LT-40 using 61 size engine (which mine does) will have more punch in windy days unlike a plane fitted with a smaller engine. Planes with smaller engines will be blown about more by windy weather.