Hmmm... the speeches do seem similar, as they're both warning Ophelia away from Hamlet. While Laertes speech is affectionate, and Hamlet's cruel, they're both basically saying the same thing - Hamlet will use her, and desert her, and she's better off without him. What I'm not sure is the motivation behind the speeches. While Laertes genuinely believes this, I don't think Hamlet does. It seems more like Hamlet is trying t push Ophelia away any way he can - and this bitter warning seemed like the best way. But I don't know if Shakespeare genuinely believed these sentiments; none of his plays feature these sort of themes, though no doubt it was a popularly-held belief at the time. (I'm getting flashbacks of Mary's "female virtue" speech in P&P).