Al,
Cortez was badly badly by the Browns TE for six last season. I'd rather let Shazier cover Gronk as he's faster, more physical and has better confidence. The last confidence part is strictly my opinion, but last season Allen lost his.
The starting player usually gets more snaps, and sets the tone. That is why I care who starts. IMO Harrison should start, as he's the better player and has leadership qualities.
Shazier can't follow Gronk around the field...the Pats line him up at WR often, in that case he needs to be treated just like a very large WR.