To be honest, the Winter War was kind of pathetic. Nobody really got anywhere.
Harribel was defeated by Aizen, Baraggan was defeated by somebody who isn't even affiliated with the Shinigami, and Starrk just kinda went blergh and was defeated. And then Aizen went along and decimated the Shinigami.
The Blood War has been really good so far. The Quincies clearly overpower the Shinigami, and they're teaching the Shinigami important lessons in not relying on monumental power to defeat an opponent.
Plus the Vandenreich are overall better characters. Harribel was the most boring character in existence, Baraggan was okay but still really blasé, and Starrk was again better but he was just so boring and was pretty much a copy-paste of Shunsui with a little girl added in to add some more depth to his character. The Vandenreich however have some really interesting characters, such as Aes Noedt and the sadistic Bambietta. The rest of 'em I'm not hugely into but they still all have great character design from what little we've seen of them.
Also the Blood War has given us some good character deaths. It's rare for Kubo to actually kill off a character, so it was good for at least Chojiro to die. Even then, Kubo didn't let him go without doing a bit of character development after the fact. And then we have Kira who may be dead, Yamamoto's death (it was
okay - at least the lead up was epic), and Unohana's on the way. Unohana's may not be a direct result of the war, but it's to further the Shinigami's war power so I'm counting it.
I voted to wait until the end to see, but so far the Blood War is definitely in the lead. The Winter War was just a display of power for Aizen; neither the Shinigami nor the Arrancar got anywhere.
EDIT: Just realised it might seem like I'm trying to bash Harribel, Baraggan and Starrk. I'm not. They all have aspects which make them good characters, I just personally find them very boring when compared to other characters