I hear sometimes a longer version (reading fully the ending '-tary), and other times a version as if ending in '-try'. Why is that? Are these choices a matter of dialect? What other examples can be given?
I would say it as "com-en-tree".
This is not so much a dialectic issue as it is accent. It's just one of those things, you will get a more strongly vowelled sound if someone is speaking with an accent closer to received pronunciation.
Lower, more working class individuals are more likely to omit the "ary" sound and instead substitute this without the vowel, producing something that would sound like "commentry".
Lower classes are often associated with informality and speech is much more lazily produced as a generic character amongst these groups.
Linguist Jenny Cheshire suggests that less pronounced pronunciation like this amongst adolescent boys, compared to girls is indicative of the men seeking covert prestige. This is such that males are already in a higher position of authority in society than women and thus demonstrating less refined pronunciations indicates they do it because they can. Not my view, but putting it out there for reference.