So which Boon from Shadow of the Past would you pick? While I love Gildor, I'm going to argue for Mr. Underhill for a couple reasons: The first is that you know you will be able to use it. Second, it can get you out of a bad initial start. Third is that it allows for greater risk taking in when it comes to determining who will quest and who will be on "Hide Duty". Lastly is the just plain cool factor attendant with a Hobbit using a using an old-school cover identity to outwit Sauron. (with all due to credit to his pointy-hat wearing case officer) Conversely it's chief drawback is its quantity (one-time per game) but I think this is substantially mitigated by its quality...if you are using it you are probably saving a hero's life.
As for Gildor, his chief benefit is that he could help for several rounds. His secondary benefit is his indirect ability to cancel an encounter card when he is drawn. (Did someone say Gildor's Council?) But his drawbacks are significant, particularly the possibility you may see him late if at all. He also requires a steady paycheck potentially complicating Mr. Underhill's--er I mean Mr. Baggins's--ability to use the Ring or pay for cards, esp Frodo's Intuition. Lastly, he effectively precludes the use of Gildor 1.0, a very useful ally. I admit that the idea of unexpected aid first pioneered in Massing is exceedingly cool, but I think when looking at the ability of each to help you get to Rivendell, Mr. Underhill is your guy.