The numbers.
2012 Mercedes SLK 350 - 3397 pounds, 302 HP, 273 lb-ft torque, 7-sp automatic transmission, convertible hard top, $54800, 0 to 60 in 5.6 seconds
2012 Porsche Cayman S - 2976 pounds, 330 HP, 273 lb-ft torque, manual or automatic transmission, coupe, $65,500 with the automatic, 0 to 60 in 4.90 seconds
Porsches are every bit as luxurious as Mercedes these days, without the incredible bulk (an extra 421 pounds, in this case). Clearly the Porsche slaughters the Mercedes in both acceleration and handling. Of course, you get what you pay for, and nothing Porsche is inexpensive.
Go with the porsche if you want mostly power and no real luxury, the mercedes will keep you in style, with extended luxury and just as much power, id go see both cars, and let her decide.