RE: 2 stroke VS 4 stroke
I just bought my first 4 stroke and love it, it is awesome. For me the 4 vs 2 stroke depends on the plane. For a small combat plane and small 2 stroke is the way to go. For a trainer or general sport plane, again 2 stroke is great bc they are economical and a breeze to setup. But if you want a warbird or a barnstormer biplane a 4 stroker is the way to go. Also, for awesome 3D a profile with a 4 stroke is incredible.
For the decathalon a 4 stroke would be great bc of the more scale sound. Although you could save a chunk of change with a 2 stroke. (For $80 you could buy a TTpro 46 and it would fly fine or you can spend over $200 for a new Saito 4 stroke)
For me it depends on the kind of plane and funds available.