Is bike theft that much of a problem that someone would actually cut down the tree to get the bike? If they can't get the whole bike, maybe they'd rather just get parts. In any case, I'd say the light pole would be safer because it's more out in the open where people will notice if someone is messing with it. And from what you say about security, it would probably be on camera there. Plus, if you do get to be known as "the guy who rides his bike to work", which I have a feeling is unavoidable no matter what you do, then the person who observes someone else messing with your bike is more likely to know who you are and call to tip you off. (I've had people call me to tell me I left my blinkie on.)
Are you uncomfortable asking your boss what he or she would recommend, or who else you should ask to find out what would be acceptable? Generally there are people in charge of the facilities who will probably eventually get in on the discussion one way or the other anyway. Might as well start off the relationship on the right foot by asking their preference.