I can only compare chocolate, for obvious reasons. Select is more of a true milk chocolate and Modern is more of a fudge/brownie batter chocolate. Select is a lot thicker, so if you enjoy thicker shakes, go with Select. Both are very macro friendly and have great profiles. Honestly, if consistency is not an issue, I would suggest sampling each one.
If you're looking for more "exotic" flavors, go with Select since Modern only has your two basic flavors so far. Select CPBC is amazing, one of the better flavored PB protein powders out there. The PB really shines. Still kicking myself in the butt for only getting 3 during the insider.