Have to agree, most of the dive market looks for PADI professionals. If you'd like to be flexible as a dive pro, PADI is the way to go, and you would probably want to consider continuing on to AI or instructor, as it makes you more marketable as a dive professional.
If you have a specific shop in mind, and they have a specified agency preference, then go with that. I believe all the major agencies provide good training, and it's really about how much of yourself you put into it -- taking the responsibility seriously, as well as really enjoying what you do. If you don't love diving, you won't make a good dive pro.
Having said that, Walter makes a good point; ask yourself why you want to take the step to divemaster. Don't think that it's necessarily the next logical step after Rescue Diver (or equivalent); take the step because you love to dive, and love helping others get the same joy out of the experience.
That's why I teach diving, even though I have a day job; I love seeing my students' faces light up at the experiences they have underwater, and to know that you helped them get there, it's a great feeling.