I personally don't like either.

First thing I do in Firefox is set it so I have one close X at the far right end of the bar. I realize this does require it to be fairly visible which tab is current but I still like that this button is always where I want it to be. My mouse always heads to the same location to close any tab, I can close multiples by clicking multiple times in the same place, and I don't waste space on every tab with X buttons when I could be seeing more of the tab title.

Just my personal preference but feels far more efficient to me.