A good question.
Both seem to have excellent high altitude performance needed on recon bird. But if I had to pick one, I would go with the Mossie. Here is why:
1. More developed than the Dinah. Multiple marks, multiple varieties. Engines increased in power as did the speed. Dinah was a one off in terms of engines and development. Probably more of a expression of what Japanese industry was capable of rather than the machine itself.
2. Seems, (and this is just appearence), seems that the Mosquito is stronger, more sturdy.
3. Later marks of Mosquitos were faster (by about 40mph) but that is not really a big deal. If you are high enough and going over 300mph, chances are an intercept is not going to happen if you are taking off to chase it.
All in all a tough thing to choose. Both very good airplanes.