Ok first off, Miettinen is a LW'er, though I vaguely remember him being able to play all 3 forward positions as well. Either way, Antti Miettinen is a superior prospect. From what I know about Lessard is that his size is more of a problem than it is to Miettinen, he isn't as defensively comitted and simply not as well-rounded. He likely does possesess a better shot.
I expect both to get a full time job with the Stars this upcoming season. Lessard will likely put up more points, but won't be as complete and thus not as valueble as Antti Miettinen IMO. Based on an 82 game schedule, no injuries, I would expect the following stats;
Antti Miettinen; 10/15 goals - 20/25 assists for 30/40 points
Junior Lessard; 15/20 goals - 15/25 assists for 30/45 points
The difference is more due their linemates and role of their line in general, as Miettinen's all-round game makes him also suitable to play on a checking line, whereas Lessard's game is either a scoring one or none at all.