Of course I would pick Van Persie over Welbeck, as like you said, he's a better finisher. One chance and you would expect RVP to put it away. However, I feel you've misinterpreted me, because I feel you're comparing individual abilities rather than how the both play as a partnership.
You said you don't rate Welbeck as a finisher. To me, I feel this is irrelevant when judging how well a partnership is together. What you should be judging, in my opinion, is the amount of passes the make to each other and link up play. We all know Van Persie, like I said before, is a better finisher than Welbeck, and on individual ability wise, Rooney and Van Persie are a superior to Rooney and Welbeck, and would most likely scare more defences than the latter. However, I think as a partnership, Rooney and Welbeck cause more trouble and link up a lot better.
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