I remember both fled to the WHA and thought Tardif was the more skilled of the two. My memory of Houle was on a breakaway and tripped over the blue line when he was with the Habs, the things you remember as a kid
Houle was the more versatile player, who could play in all situations. He was never a scorer but whose superior skating and hockey I.Q. allowed him to largely shut down Hull in that memorable 1971 series.
Tardif was largely a one dimensional/stationary player, who had superior scoring ability, a bit of size but was rather mediocre in all other facets of the game. But with the Mahovlich brothers, Shutt, Lambert, Lefley, Gainey and Murray Wilson (all of whom could or did play left wing) already on the team or who were about to emerge, the Canadiens rightly decided not to engage in an expensive bidding war for a player (Tardif) that was easily replaceable.
Not retaining Houle, was clearly a mistake. A mistake that was rectified when the Canadiens re-signed Houle for that glorious run of Stanley Cups in the late 1970's.