I'm looking at either a Sig Something Extra or a GP RV-4.
Either one will be powered using an OS FS .61 for power.. why? because it's there..

From a building perspective what kit is more fun to work on?

I have built and fly both. I will say that a .61 is a ton of motor for either plane. The RV flys very well on a .46 and mine has flaps, neat extra toy. Both planes have soft landing gear and could use an aftermarket gear. I would think you would need to keep all of the weight towards the tail in either design to use the .61. Keep us posted on you build, it will be interesting to see how you fit this together, and pics would be great! Good luck.
JEB

Yeah the .61 is a bit heavy but it's a great running 4 stroke.
I have an OS .45 FSR but it's in my old Midwest Sweet Stik.

My other motors looking for a plane to pull are an OS FP .40, OS Max-S .35 and a Saito FA-45 MK II.

The planes are so different its almost impossible to compare the two. Without a doubt the building time for the Sig SE will be a lot less not only because the SE is a simpler design, the kit is better. The parts are all laser cut so very little sanding or fitting is required and like sig says you can build it as fast as you can glue it together. My Sig SE for the short time that I had it flew great with an old OS .46SF. Although I have built several GP kits I have not built the Rv-40. I finished the GP Mustang 40 this winter and while it was a easy build it took much more time than the Sig SE. The wood quality will much better in the Sig kit. I have thought about building the RV-40 I think could be a real nice fun scale project esp if it was finihed with a scale color scheme. I think bsp got confused with how you named your engine and thought you were wanting to use a OS .61SF 2-stroke a OS .61 4-stroke should be good power for either plane. You will probably need to buy or make a taller landing gear for the Sig SE if you want to swing a prop with a dia of more than 12"