As others have said, it does depends on the type of trails you are going to be riding. Tight, slow, technical, trail riding rewards smooth, low end power with more compliant suspension setups. Wider, open fire roads, sand pits, desert areas, are happy with snappy/more power and stiffer suspension setups.
Rider ability is important also. Better, faster, more aggressive riders generally require stiffer suspension settings and can more easily adapt to more abrupt power response.
Four stroke vs two stroke. Lots to talk about there (but, I won't). Personal preference, maintenance cost/difficulty, power type, current trends. It's your choice.
For tight, slower, trail riding a 250/400 four stroke or a 250 two stroke would probably be okay. As the areas open up and get faster the larger bikes are more fun.
I have a 2001 Honda CR500 (two stroke) and my son has a 2004 Yamaha YZ450F (four stroke). Riding in tight, slow, technical areas are not either bikes element and you will suffer. Riding in more open/faster areas and WWWWWEEEEEEEE!!!!! - lots of fun.
What does your son ride? It makes a difference. My daughter has a 2003 Yamaha TTR125 (four stroke). When we ride together it is often difficult because of the contrast between the bike/rider combination (she is a novice with a slow, mellow bike and I am of intermediate ability with a fast bike). I have considered buying a smaller, more mellow, 250 four stroke so I can ride with her more comfortably.
Another consideration, you are a Busa rider. You obviously appreciate/enjoy powerful motorcycles. Buying a bike that doesn't have some at least some snap might be disappointing.
IMHO look hard at the 250 two strokes (yeah, it's bucking the current four stroke trend - except for KTM - but the 250 two strokes are very flexible). Light, powerful, can be tuned (engine and suspension) for tight trails or open areas. Plus they are lots of fun.
Good luck in your decision.