Both are fantastic DH tire choices, I've personally ridden both extensively. I'll break it down like this:
Minion: If you live somewhere wet, muddy and loose I would run Minion DHF on front AND rear. I've done this many times, the DHF runs great as a rear and helps significantly with loose, muddy and wet conditions. Minion DHR2 is a great tire with more grip, better breaking then the Highroller but will run slower, use only as a rear.
In short - Minion will provide increased grip and breaking power
Highroller: These tires run faster along the centre line, they perform better in dry, dusty and fast conditions. The side knobs are aggressive enough to handle hard cornering and wet conditions. I would run Highroller 2s front and rear (I've been doing just this for the past 2 years).
In short - Highroller 2 is a faster rolling tire that rides with a floaty and loose feeling
Both tires come in 60a compound and Super Tacky (S/T), but I would only recommend running the 60a compound. S/T is meant to provide better traction but they wear incredibly fast, about twice as fast as 60a. I always found that if I needed more grip I would run Minion DHF front and rear in 60a compound, this will give you more grip then any S/T tire.