The difference in flame diameter between a Bunsen and Meker (or Fisher) burner really isn't that much, so I'm not sure how much larger the sterile field would be. On the other hand, Fisher burners can be tuned to be much hotter than standard Bunsen burners, so that may impact the size of the field as well. But, at the same time, they use more fuel, so you have to think of that as an additional cost.
If you're just sterilizing inoculating needles/loops, flask necks, etc., a Bunsen is perfectly fine. However, if you're trying to create a larger "sterile" field, there are too many variables to make a definitive recommendation. Are you setting up in an area with relatively little airflow, or are there cross-breezes? How large of a field do you actually need - what is the area where you'll be working with open containers of sterile media?
All in all, to be safest, you'd probably want the Fisher burner, but I don't know what the cost differential is and what your budget is. If you're careful with media manipulation and have a good still area to work in, a Bunsen should be fine, though.