By far the most wind turbines have been built on land and for good reason: it's much cheaper. As long as there is space/not too many people complaining it makes more sense to build on land. Not sure exactly but on land installed wind is probably ~10x the installed capacity offshore. However on land turbines are pretty much maxed out at about 7,5MW, due to logistical difficulties of getting larger blades, towers and nacelles transported over the road. These limitations don't apply to offshore sites, so the up-scaling continues. In the last few years typical offshore turbines have grown to about 9MW, but that is not the limit. Within a few years we should see 12-15MW turbines being installed, which should bring the cost down. Combined with the better wind conditions offshore (at least around the North Sea) offshore wind may become cheaper than expanding on land.