Clockwork Prince was even better than Clockwork Angel in my opinion. There were a lot of questions left unresolved in Clockwork Angel, and Clockwork Prince answered most of them, so no frustrated grunts! In this book, there was a lot of "nothing is as it seems" going on. A lot of background info that is reveled about Will, Tessa, etc , and it's realllll juicy.
My heart broke into teeny tiny pieces while reading the book, mostly for Will, and I had to take a few breaks to pick up the morsels (the first time was right after reading the epilogue...). In that sense, the book is a bit more romantic.If you couldn't decide whether you were team Will or team Jem, you'll probably figure it out by reading Clockwork Prince.
The book was a little less action, a bit more character development and understanding why they do the things they do. It called for a brilliant ending, that shattered the already shattered pieces of my heart into teenier tinier pieces. I loved Cassandra Clare's imagination and creativity towards this book, remember folks, you don't need to read The Mortal Instruments novels (though I recommend you do because it's an extraordinary series) to get on with The Infernal Devices.
Clockwork Prince was my favorite book out of the two, it has solidified my fangirlism for The Infernal Devices. I'll be gluing my heart back together as I wait for Clockwork Princess, on sale September 2013 (OUCH)