Both ways of notating a roll are correct.
I, myself, prefer trill notation because its easy to visualize.
In most pieces I've played the notation has been trill. I would say about 55% use trill and 45% use tremolo.
However, if you are writing for an inexperienced performer I would use tremolo because younger player are more used to tremolo like most snare rolls are notated with.
As to the length question, it may be better to carry the end to the downbeat. When you roll on a timpani,it's all about fast, steady hands. (I try to move about 32nd notes if it's not too fast) It's very hard to stop on the last 32nd note of a roll, (it can be done, but it's hard).
Hope that helped.