I will say this was really difficult for me to make sense these charters are similar and one of them is voiced by Tara Strong my all time favorite voice actress. BUT am going to try to be unbiased as I can.
 
Personality: There Personality are Similar trying to make everybody happy  
Design: Pinkie Pie is has a Good Design with her pinks and with her main and her cutie mark suits her perfectly as for Unikitty I find it cute with her shades of pink her little horn to represent her name and having red blushing cheecks.  
Development I had to give it to Pinkie cause Pinkie has gone threw a long way. As for Unikitty she still in development(Which ill update this in the near future)  
Entertainment: They are both enjoyable and very fun/funny
 
By a cotton candy hair Pinkie Pie is the winner
 
Meme belongs to Stocking-Star
 
Blank meme here:Character Vs Meme