I personally don't like wool because it dries too slowly. It's not a huge issue when we are talking about a thin t-shirt though, so it shouldn't be a huge deal. In summer, you are not going to get the instant cooling effect that you get from a synthetic t-shirt because the evaporation time is spread out over a longer period. That can actually be a good thing. In winter, when it's harder to dry out clothing, I prefer synthetic base layers because they dry faster. Cotton is probably not that great of a choice. For the money I would rather go with a simple synthetic t-shirt. If you have the extra money, then you can play around with wool and synthetic shirts and see what suits you better.
+1
Merino sheep farmers must have the best advertising execs in the world, it seems to have made it into the main stay of most activities now, even ones where it's really not suited.
In my experience Merino is great for low level activities, once you start getting to higher levels of activity though it's a terrible choice.
Plus, although it makes good material for pullovers for low activity levels it's by far the worst material i can imagine using for areas where we sweat a lot like socks and underwear.
The problem is the material just doesn't wick, so you end up sweating say under your arms and instead of that sweat spreading out over a larger surface area to aid evaporation, it just stays where it is and gets wetter and wetter from sweat.
To the point where i've had some toes that looked like dried prunes from the sweat to others that were as dry as a bone.
The thinner the merino the less of a problem it becomes BUT once you start getting below 260wt longevity really starts to become a problem.
There are some interesting material mixtures around these days but still if it's warm and dry i wear cotton tops, if it's wet and cold i'll wear synthetic for high level activities like hill walking, running and MTBing.