Set up as new or restore from backup

I had the common wi-fi issues (wi-fi grayed out) as most iphone 4s customers, so I had backed up all my personal data and sent my iphone to Apple.

Luckily my purchase was inder guarantee, so Apple replaced my iphone with a brand new one.

Can anyone tell me what should I do now to activate the new iphone? Should I set it up as a new one or restore from backup from my former iphone?