Can I ask why on earth you'd want to upgrade to iOS 7 if your running 6? The 4S will run iOS 6 very, very well. The 4S ran iOS 7 like crap, except 7.1.2 (which isn't available anymore). And even 7.1.2 could have been miles better.
Keep it on 6. Besides the look of 6 is much better, IMO.