Congratulations!! What an awesome problem to have!
Hmm...these were the reasons why I went with the agency I did:
1. Enthusiasm. I was met with not one but two agents at the agency, both of whom were super enthusiastic about my book. They even said that they'd sent my book to their New York branch. I loved that they were so tight-knit as an agency.
2. Their previous sales. Strong domestic and international sales to big and small houses. Yay!
3. Their comments. There were a few pretty major changes they suggested, and I agreed with almost all of them.
4. Level of comfort. I felt very comfortable talking to them and bringing up concerns etc. This is so important because with my last book, they loved the concept but they suggested a major overhaul (turning it from a stand-alone to a series), and they pretty much held me by the hoof and took me through the revision process step by step, which brings me to the last point...
5. How much editing do they do? How hands on are they with your books? I need an agent who patiently nudges me forward every step of the way, but some writers prefer agents who leave them to themselves. It all depends on you.
Good luck, and I hope you make a choice you're happy with!