I dunno, it is hard to tell because one is a fleshed out and interesting character and the other is just kind of an enigma, from what I have seen they both seem pretty self-serving with their own hidden agendas, G-Man seems more like a contractor, hiring out people who make things happen to whoever suits his agenda, Illusive man however is shown as the ultimate transhumanist, many of his motivations go untold, but there is a much deeper character there to get to understand, however his goal at the end of the day is always a pro-human message so not all that bad.
I trust G-man less because we don't know nearly anything about him as a character, he is just another factor in the half-life games, which to be honest, I didn't really find all that compelling anyway, mainly due to the main character being a mute with no means of expression and is just expected to be the hero of whatever random event has happened, who knows really?
Ah well, that is a point for another day (IE when you can make a thread/post that does not portray half life as being the pinnacle of modern gaming and not get torn apart by the PC crowd on here...)