Both have good points. I got the Milwaukee 1st and have been using it for the week. I like it. It flips open effortlessly. It's a one handed utility knife. Grab it off your belt and you can open or close it with a one hand flip. The clip is soft but holds nice, easy to put on or take off.
Holds one extra blade according to the manual, I can fit two. When it's in the open position it is pretty stable, but has a slight wiggle if you're looking for it. The blade itself is tight but still moves a bit. The blade lock seems to hold the blade nice and inserting or removing a new blade is easy.
It locks in the fully open and fully closed position.
I just got the Irwin today. Came in the mail on a Sunday . I really haven't used the knife at all, just playing. First thing I noticed is the blade swing mechanism is to tight. You can't flip it, makes it hard to open. I squirted it with WD40 and it loosened up and now it's able to flip open with a flick of the wrist. Not nearly as easy as the Milwaukee. Like I said, I haven't worn it so I can't really comment on the clip, seems tight so it'll probably need to be stretched out so it will go on easy. When the knife is in the open position it is very stable, almost no movement. The blade itself has about the same movement as the Milwaukee. Holds two extra blades.
It locks in the fully open but doesn't lock in the fully closed position.
It has a special feature that only allows you to remove the blade when it's open at 45 degrees. So no matter how much force you're using on the knife the blade won't get pulled out.
I can say I've had this problem with all the flip utility knives I've owned. Knife coming out, especially when cutting up a lot of cardboard.
The Irwin also sports a screwdriver. It's a hex holder so you can put any 1" screwdriver tip in it you want. It came with a #2 Philips and a #2 Robertson.
Got the Milwaukee at my local HD for $14.97
Got the Irwin on Amazon for $17.99 and $5.29 shipping