It is not that they are "better" but compared to the stock 205/50-17 the 215/45's are wider and have a slightly shorter side-wall possibly yeilding better turn-in response.
It should be noted that most 215/45's (or at least every one that i've read the specs on) reccomend at least a 7" wide rim. This may mean unless you are buying a tire with a super stiff side-wall that they may exhibit the undesired behaviour of turning over onto the sidewall during extreme cornering...
The 215/50's have a slightly taller side-wall than the stock size but are O.K. to mount on the 3's 6.5" stock rims.
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