I think I'd go with the lower speed at this point. This is my first elevon plane and first plane after aileron trainer. I'm willing to cut out a little space for a bigger prop if necessary. Up to a 4 inch, that is.

The eflight motor is actually a little bigger than the stock motor. Stock is 12mm by 30mm and less than 20g. the eflite is 20mm by 40mm and 68g. Is the weight difference taken into account?

Also, I'm assuming that the speeds you are giving are at full throttle. Am I correct in this? I usually fly at half throttle or less. I typically overpower the crap out of my planes. It helped a lot when I was starting out to get out of sticky situations. It was weird having to go full throttle with the stock setup. It sank like a rock as soon as I gave any aileron input (yes, I gave elevator as well).