Which method of preparation would produce a more tender and juicier steak, grilling or broiling?
If by 'grill pan' you mean something heavy (cast iron) like:
 OR

then it would be your choice as to whether you want to heat them
on top of your stove or under your broiler. If your 'grill pan' is less heavy duty (say aluminum) then it is unlikely to be capable of with standing the heat required to 'best' cook a steak. When the weather here does not favor outdoor grilling I get great results out of placing my cast iron griddle/grill (similar to the second picture) inside of a half-sheet pan and under the broiler to preheat. Once the iron reaches 500F I will add the steak for about 3 minutes per side (for a med-rare) I would also suggest wiping the iron with a little oil first.