I highly recommend reading this thread here.
It goes into discussions about the different stats and such.
It's a great read with good information.viewtopic.php?f=2&t=688&p=5479#p5479
The more I've done pet battles though the more refined my decisions about pets and stats have gotten. Pets that will fill primary roles of tanks and healers like the crabs or fawns I'll actually go the the healthier ones. If I'm going for a pet to counter a specific family type I may go for the stronger type. Generally, I'm still all for the faster pets. I also notice that sometimes the "balanced" breed of a pet may still have one high stat. I'm not sure if they even all over by the time they are 25 or not.