Hand for jib sheets
The most important consideration is your winches. Some self tailing winch's jaws on boats of your size range will simply NOT handle lines any smaller than 1/2 inch. We've had a number of our C34 skippers change to just slightly smaller, stronger line, and they report that their winches will not handle the smaller line. They kept the line and need to do extra wraps on their winches, which is a PITA.
We still use 1/2 inch lines (StaSet), they're "big" but I like them. We rarely have "light" winds here in San Francisco, so they work fine most of the time. When the winds are light, the jib sheets are heavier than the jib can handle, but I've learned to live with it.
Your winches might be smaller, and thus not an issue, but check it out first before you invest in a lot more line that won't work with your winches.
How can the blocks (fairlead blocks) be an issue if what you have works now? If they bind (heck, you didn't mention that, so they shouldn't), and you like the 1/2 line, then buy wider blocks (not that you'd need to if they work now). Compare the cost of the different kinds of line that are available and blocks if that becomes an issue, but suggesting that the blocks are a problem isn't a question you asked, nor seem appropriate.
The year of your boat would be helpful in either your sign on or profile. They've made C30s for a looooong time.