SizeOn!
I like SizeOn better. I actually think SizeOn taste better as well (Lemon flavored at least). I enjoy sipping on it during my workout as it seems to fuel my tank during my workout, and the pumps you get due to the carbs/sugar in it are unreal. I felt SizeOn working on my 2nd day using it, and set after set, I just wanted weight! It really made me a mad man in the gym, I never felt that way before.
I'm about half way through the tub and I have awesome endurance, though the 'shock' of being able to hang w/ heavy weight has subsided, I'm still able to do the heavy weight for more sets... its just not new anymore.
I think GreenMag is a very good product as well, and I use it on a cut cycle due to it not having the carbs/sugar SizeOn has.
Results dept... honestly, its hard to say. They both have 5g of high quality creatine in it, but I just 'feel' like I get more from SizeOn... perhaps w/ the extra carbs and absorption?
Both are great, but I give SizeOn the slight edge overall. If its all about the dollar, then GreenMag.