The LC is an arguably better bike.
I had two (3 in total) RD400's because of nostalgia, I wasn't looking for the best bike, it was about a specific model and colour. Not sure I'd have another as I've got nostalgia out of my system!
All power valves can give problems but nothing major. Usually they seize up. The RD400 has simple reed valves on the inlets.
Also, the RD250 isn't bad either and there are loads of 250LC's converted to 350 so you need to check all the numbers if you are paying decent money. Ditto the 400's. Most buyers want matching numbers so that makes a big price difference.