JMO, I'd stick with Beckett. He is learning to be a pitcher and not just throw heat, he's just been too young and thickheaded to get it. He's ultra competitive - he will learn to mix better and throw more pitches as he gets older and begins to lose some zip for that same reason he has challenged fastball so much in the past.
Aside from Bruce Hurst and Bill Lee, who do you remember being a dominant lefty starter at Fenway before Lester came into his own? (Actually, you are too young to have seen Lee and were probably like 8 or 10 when Hurst retired, correct?)