S2 for sure.
Its an Android so you have a world of apps at your disposal, a dual core processor and twice the RAM of the Lumia, a larger screen with SAMOLED, expandable memory (trust me, 16gb fills up soon these days), video recording at 1080p rather than Lumia's 720p, a front camera so you can skype/fring with friends (none in Lumia), better battery life and slimmer+lighter.
I've always been a Nokia fan but Nokia is struggling to catch up these days, which is just sad.