That really depends on wether or not you're the type of guy who absolutely must always have the latest phone, and will feel inadequate with an older handset. Considering you are still rolling with a 4S, then I guess you're probably not.
There's always going to be a new one on the horizon. I just changed from a vagrant-spec iPhone 4 to a 5, and I don't regret it. If a 5S was to be released tomorrow, I wouldn't be bothered at all unless it came with a Teasmade. A new form factor would obviously be a bit of a bummer, but even then...I rolled with a 4 for long enough, never upgraded to a 4S, and didn't get too excited about the 5 when it was released, so I'm sure I'd deal with it.
Having said that, it's a tough call to make. Really, only you can decide if you'll be bothered about the phone being upgraded in a few months. Personally I don't think I'll care, but if I do, then I'll sell the 5 and get the new one. No big deal. I roll SIM only though, so I have flexibility with handsets. If you're gonna be tied to a 24 months contract, then it would possibly be wise to wait for the next phone.