I don't know about the HERMS coil because I don't have a HERMS, but for the chiller, the higher your flow, the faster it will chill, so 1/2 is better than 3/8. Even if you only go with 3/8, 50 feet will be long enough - in fact, you would probably be better with two 25 foot sections in parallel than with a single 50 foot section - you will have less resistance and thus a higher flow rate. If you have a 100 foot, 3/8 chiller, the resistance in the line will really cut down your flow - if you have to have 100 feet, make it two 50 foot sections in parallel.
Just as important as flow rate for quick chilling is to keep the wort moving past the coils - either by manual or mechanical stirring, or by using a pump to recirculate wort and inject it at the top of the kettle to keep a whirlpool going.