From what I know of the GDL, it doesn't really make a difference. What is important is that you pass it, and hopefully pass it well. I would go on which city you prefer and cost to be honest, though do look at any difference in examination methods as these can make a real difference. Open book exams, use of multiple choice tests or coursework may suit some people, others may prefer the traditional 3 hour exam for each module approach so do look into any differences.
Also beware any marketing crap either give you. Wherever the GDL is concerned there is a real attitude of money grabbing and over-enthusiastic marketing. Just keep that in mind.