The draw of a McLaren vs a Porsche might be compelling - Formula 1 heritage + race cars + sports cars. This certainly appeals to me.
For my likes, Porsche has become too soft, too PC, too worried about C02, and much too focused on having three models for every possible market niche.
Sorry to sound so critical, but if it were me, I'd skip over the 918 with the Cayenne/Panamera inspired interior for a McLaren everyday of the week.