I've flown my WM Intruder in 2 SPA events thus far and it has represented itself in a respectable manner. The plane is a solid flying model with no bad quirks. Put the CG further back than the plans call for and you will love it. I started with my CG about 1/4" in front of the main gear, and eventually moved it back to the main gear.
The plane doesn't seem to be overly sensitive to control surface throws. I've got max throws on my ruder, and my elevator throw is at 5/8" up and 5/8" down. My elevator doesn't seem "touchy" at all in the air, yet it provides for great control at slow landing speeds. BTW.. this plane has a fairly thin wing and stab so she's pretty efficient in the glide. She's not a lead sled so there's no need to treat the landing pattern like that of space shuttle! A relatively flat pattern will do just fine - use the last 1/2 of the final approach to ease the nose up and she'll establish a very pretty "nose high" descent rate and a main gear 1st touch down. You'll have trouble if you're one of those that likes to dive a plane down to the runway.
As for the .61 2 stroke vs .91 4 stroke, all I can offer is an opinion. When it comes to vertical performance, I'm of the opinion that a .91 4 stroke is going to have an advantage over an unpiped .61 2 stroke. I completely agree with the SPA's decision to not allow tuned pipes or retracts as it makes for some very uncomplicated, affordable, and competative flying. The Intruder would certainly fly novice pattern with a .61 2 stroke but I don't think it would do well with some of the more vertical maneuvers such as the figure M with 1/2 rolls. Your milage may vary, so the .61 2 stroke may be your cup of tea
The WM Intruder 90R has proven to be a great aircraft for me. Because of the SPA and the WM Intruder 90R, I am able to enjoy some serious pattern competition on less than a shoe string budget. BYW- I fly a very basic Futaba 6EX FM radio with 4 std 3004 servos. I'm using an older version O.S. .91 surpass w/pump. I've not had a single minute where I wished I had made a different choice regarding the plane, radio, or engine.
Get a Truder, join the SPA, attend an SPA pattern contest and you'll have move fun than a barrel of monkeys!