Actually neither way seals in the juices. Cooking a thin steak, burger or poultry, quick searing is the best way to cook. However, cooking a thicker piece of meat IMO the reverse sear is the best way to cook all large cuts.
Here's why..........if you cook a thick piece of meat using the "Reverse Sear" method you are slowly increasing the internal temp to begin the cooking process, therefore not forcing the internal meat temperature to increase/cook and start to steam out. Once it's time for the sear your meat is 3/4 of the way cooked (100*-110* internal). So your meat is now "tempered" and ready for the quick extreme heat of the sear without the SHOCK of the sudden heat and will not be exposed as long as it would if you seared in the beginning. This method will give you a nice uniform doneness all the way through, versus the more commonly seen "Grey outter edges with a rare center"
For anyone who thinks you can "sear the juices of a piece of meat in", try this experiment and you will see you cannot no matter what method you use. Cook a steak to your desired doneness (med well or well will not work cause the meat will be try at that point) using whatever method you want and using tongs to handle the meat. Remove the meat an place it on a dish and let rest for 10 minutes and you will have a puddle of juices PERIOD. If the steak was seared and the juices were sealed it, the plate would be dry.
Another misconception is if you poke a piece of meat with a fork all the juices will run out. Again, not true. The juices in that very small immediate area will, but not in the entire cut.
