Left: EG: Summertime Shorts. Right: Equestria Girls movie screencaps. DJ PON-3 image is from Friendship Games.
 
This is a comparison of the animation and style used in EG: Summertime Shorts and the Equestria Girls movies. We have all already noticed that the animation and style used for both are different, but not totally. The main change is in the Humane 6 and the CMCs; several of their strokes are thinner, as well as several pointy details are now round. Not only that, their colors are bad! This is most noticeable in Rarity, Fluttershy and Pinkie. All of these details can be checked looking at the title card of the short Steps of Pep, which uses Pinkie puppet made by DHX Media (original colors and details). Boulder Media also used the DHX Media puppets for most of the background characters, but those of the Humane 6 and CMCs have been changed for no apparent reason (which in my opinion, DHX puppets are better).
 
Regarding the animation, I think Boulder Media is better. From my perspective, it is more fluid, and other details are added that DHX does not usually do, such as illuminating the characters in the Formal Fall and some smear frames made from scratch.