Is the speaker broken, or are you trying to upgrade? Without knowing more I would say for you go OEM. If you aren't going to upgrade anything else, then you'll get very little, if any improvement from simply swapping out the lower door speaker. Any aftermarket speaker will require an adapter plate that you'll either need to buy, or make. You'll have to drill out rivets to get the old speaker out.
Definitely replace both. I replaced all fronts with FOCAL for VW. Sounds GREAT.
Rears I replaced with JBL Club 625SQ, these are also great sounding! I have stock amp and aftermarket h/u