1.Giving the possibility of owning either, I 
would have to say I definitely prefer cels. 

2.Cels have color, you can put a background 
behind them, if there's multiple layers, you can 
see all of them if they're overlapping, and 
they're what you actually see on the screen.

3. I tend to buy more Dougas, because they are 
closer to what you actually see on the screen.  
Sometimes, however, if say, a Genga of a 
particular scene has a more complete image(you 
know, not missing a mouth, etc.), then I'll 
prefer the Genga.  Gengas are also cool because 
they usually have more color, and I've noticed 
they are generally more expensive.

4.  I would have to say, between those two 
choices, I would look for nicer, more "eye 
candy" sketches- that's just probably because 
I'm poor enough that I have to be 
selective..lol.