I prefer the medium and longer ones, but that depends on the distance. For example, the EC I take daily has small ones for large distances, without the pull-out tables at the outter seats. Hence, this is quite uncomfortable since most passengers want to work on their laptops and end up having it on their knees. Of course, these tables make getting in and out of the inner seats harder, but that is more of a problem for the R and IR lines.
On the other hand, small-distance trains (R, IR, etc) are perfect with small, foldable ones or even no table at all to increase seating capacity.