My two cents... My husband is the luckiest person I know and wins everything at his quarterly manager meetings. As a result of this and what we have purchased, we have about 1 of everything from Wal-Mart to commercial grade. I am not a fan of Makita. Our Dewalt and Bosch 18V I think feel the best as far as power goes and we use them a lot for major projects but seem for me a bit too much for putting furniture together. I like that the Bosch 18V just clicks and locks a bit into place easily. As others said in previous posts, lithium ion battery is best. For drilling pocket holes I use an old corded drill and it stays on the Kreg table ready without having to worry about batteries.
As far as driving pocket screws I find that about anything will work just fine. So I often us what is comfortable for me and not clunky feeling while putting things together. This is good especially when you have dozens of pocket screws in a project!! My son asked for a Hitachi 12V for Christmas. I was hesitant but it looked cool to him and it was small for his hands. Now... I use it A LOT! Shhhh! Don't tell him! It is small and has just enough power to drive the screw. I like the light also.
I highly suggest getting one of those sets (Dewalt or Bosch) that come with and 18V and a smaller one like a 12 V. I saw a lot of them at Christmas. If you are doing a lot with the Kreg jig you will certainly appreciate having two! This point was originally all I was going to say but I guess I am feeling wordy today. hee hee