I haven't seen either, but have studied both myself. With the recent $500 drop for the Epson, I'm shocked you can get a 420 for a similar price. At the same price, the 420 would seem the no-brainer decision to me.
Two full speed 18 gbps inputs is the big thing on the 420 versus Epson's outdated 10.2gbps hdmi pipe. Input switching is the biggest complaint on any of the JVCs but most people aren't driven crazy by that. If you need a projector immediately, I'd go 420, but I suspect Epson will be releasing a new model in the next couple months (okay, maybe four to six? the 5040 was announced in June 2016) to address the 5040's most glaring weakness (their chipset).
Personally, I'm still holding out, but I doubt the JVC would result in as much near-term buyer's remorse since it already has the better chipset, arguably better HDR support, definitively better blacks, and the JVC comes in beautiful black instead of that hideous, obnoxious white (lol, I can't hammer that one enough!).