I prefer I305.
See, the problem with I305 is that it doesn't behave like a mega/giga. Millenium Force behaves like a crap one. So neither of them are good at what they do and I suppose thus neither of them deserve any critical acclaim.
I305 shouldn't be 305ft tall. That's it's problem. There's absolutely no need for that stupid lift and first drop, only to spend the rest of the ride fairly low to the ground. I can only assume they didn't have the money or space to do a giga coaster properly. Like everything at KD, there's no emphasis on making a quality ride, but an attempt to get people interested with something unusual.
But that said, MF doesn't do anything. It's forceless, with no sense of speed or intensity or drama. I305 at least has that, it it's second half, when it quits trying to be what it's not.
On top of that, I305 has the vile intensity issue which prevents you thinking "that was fun, lets do it again".
Neither of them are great, but I think I305 is at least interesting with some good bits. MF doesn't have anything going for it, at all. It's dull, it feels slow, the layout is uninspired... And it's at Cedar Point.