I read these novels back to back. It seemed fitting to do it this way so that I didn't forget anything in between the two books, particularly since Divergent> is a cliffhanger. I can't give you a complete review of these books at this time without spoilers, but I will try my best.
Allegiant is the final installment in the series and is told from alternating viewpoints: both Tobias and Tris give their own accounts. I found this to be a really good difference and divergence (no pun intended) from Roth's previous two novels because, while I enjoyed living in Tris' head, I also was very curious about Tobias and his experience. Frankly, there is also so much you can take of the sort of angst that Tris has and I really enjoyed the widening of the experiences that Tobias' experiences give. It also really shows the differences between how one person, Tris, views actions of Tobias and how he views his own actions (and which are often completely different). This was generally a really great ending to a strong and respectable YA, dystopian series.
Insurgent was the second novel in the series. For me, it cemented Tris' roll as both an action hero and as a loyal and multifaceted friend and teammate. She was very strong and three dimensional as a character but also realistic in the sense that she was a character that could be someone you would know in real life. I really enjoyed this book because it also ramps up the romance between Tris and Tobias and begins to highlight the struggles they must face. Loved the cliffhanger at the end.