Delilah has an Avid chip (inserted by the shelter) and Blue has a HomeAgain chip (inserted by my vet). The rest of my kitties will be getting HomeAgain chips in the next few months. HomeAgain does offer additional services like the email alerts with an annual fee, but the basic registration fee will connect your info to the chip for the pet's lifetime. (I do not pay the annual fee with HomeAgain, and I did get Blue back from a shelter via her microchip once when she wandered off.) I'm not sure what additional services Avid offers.
I like the HomeAgain service better because you can manage your pet's "profile" on their website. You can update your info, add your pet's photo, etc. You can also register your pet's microchip yourself via their website as soon as it's inserted, versus having the info mailed off and waiting for a confirmation letter.
The last time I looked, Avid did not offer any online capabilities. I think it took over a month to get confirmation of Delilah's Avid registration, and I had problems with information errors (gross misspellings, but at least the phone numbers were right).
Just my two cents!