I'll get back to you when I'm done flipping a coin...
That's what 20+ years of using both has taught me.
Honestly, though - each has beaten the other for me, and there doesn't seem to be any rhyme or reason to my results. I put 3 .257" bullets in one .258" hole at 100 hundreds once (with a witness!) and thought H was incredible. I've used H to consistently put 7 Mag bullets from various rifles in sub 3/4" groups. Then when all I could find was IMR, one of those 7 Mags turned into a 1/2" shooter.
I used to mostly lean toward H because of the temperature insensitivity touted by the factory and others with experience. I won't argue that point because it makes sense, but IMR continues to put meat in the freezer for me - even when I've sighted-in at 80 degrees and hunted at 15 degrees. That point is hard to argue as well.
