So, I have two cylinders on my O-300 that have leaking exhaust valves. Should I buy two new superior cylinders for $789ea. Or send these off to be overhauled? Both of these cylinders have had valve work done since their overhaul, less than 400 hrs ago.

In general...if I can figure out a way to afford it...I try to buy new parts. If the cylinders have less than 1000 hours total maybe I try to get them overhauled in your situation. But if they have already run through 2000 hrs, been overhauled once or more, and are having issues again within 400 hours...I would get new ones.

I don't know how long turn-around times are in your area, but that might be a factor to consider. Myself, I did both last time I was in your shoes. Installed a new cylinder, and overhauled the leaking one to keep on hand as a spare.
Good luck,
-DP