Go with a Superchips Tuning ("SCT") based chip (Apten uses SCT now) and get a quality programmer to write tunes to your specs and desires. Bamachips (Doug) and Hypnotic Illusions (Wayne) are two site sponsors that are excellent to deal with and are both SCT dealers (Apten is also a dealer, they don't have their own "chip" per se). You'll hear Apten mentioned a lot, but their service is far less than stellar these days. I've got an Apten programmed chip and it's been decent. I recently had both Doug and Wayne write me some tunes on my chip (I can store 4 programs), so I can't comment on their handiwork yet (chip is in transit), but others like their work.
Also, don't get hung up on a chip. For a few $$$ more, you can get a flasher. Much more convenient to use than a chip which has to be attached to your vehicle at all times. With a flasher, you load the program into your computer and it stays there. You disconnect the flasher cable and still have the benefits of a program. It's equally easy to return you Ex to stock. I can speak from experience about connection problems with chips, and the adverse reaction to your Explorer's reliability when there is a connection issue.
I've never heard much in the way of good reviews of Jet stuff. Seems that you get what you pay for. Definitely use the SEARCH function on this site to scour the archives. There have been countless discussions on this topic already.
