This is one of the most common questions I get from patients. It's not easy to decide.
A common marketing campaign says you should buy a tampon designed by a woman instead of one designed by a man. This is simply preposterous. Do you really want something designed by a woman inside your body? I didn't think so.
Also don't worry about buying different sizes for your light and heavy days. It's cheaper to simply 'double-up' on the heavy days.
I recommend using Tampax. It's what I use... or I mean what I would use.