Merino or non-merino socks?
In your case, I would purchase based on reviews (for durability) and feel.  Merino have the advantage of tending to stink less.  However, if you are riding only once a week (not sounding like a commute) it's less of an issue for you.  Some people will likely argue that one or the other lasts longer, but the reality is that manufacturing techniques vary so widely that you can find good examples of either extreme in both makes.  Wool generally requires a bit more attention with care (not drying on high).  

My sock collection is about 95% wool.  I used to lean towards coolmax (or other synthetics), but wool socks now feel so nice to me that I can't see a reason to go back.  The only synthetic socks I regularly use now are my vapor barriers, which almost always get a wool over the top.