Either should do fine if planted in the right spot. The 'Mega Blue' is a bit taller and bluer than the 'Red Bull'. The only reason I know of that either would flop would be too much shade, too much water and/or too much fertilizer. Both types would thrive in a hot, sunny spot in soil (sandy or clay) with low nutrients and be very drought tolerant once established.

I like the red blooms of the 'Red Bull'. Its a hard choice.