Hi Tim,
You're going in October huh? Pretty organised to be sorting your hostel now. Good work! Both Bahaus and Metropolis are good but they are different. Travelling with my girlfriend I preferred Metropolis; it was smaller, our double room had aircon (our room in B didn't, and the view from the roof terrace was better (B's is partially blocked by another hotel). Both provide good free breakfast, late check out and airport transfer. B is bigger and has a really nice indoor chill out and breakfast area as well as a terrace and being bigger is probably better for meeting people. It depends what you are going for but I preferred M for the reasons above and the fact that our room had views of the Bosphorous! Hope this helps, have a great time - Istanbul is an amazing city. Make sure you go over to Beyoglu on the Asian side one evening for great food and buzzing nightlife. It is difficult to find many good eating places in Sultanahmet. Bye!