If you like iOS then wait for the iPhone 5. If you like Android or are willing to try it out, get the Galaxy Nexus. The Nexus phones are the Android flagships and the iPhone is the only phone that runs iOS and is thus the flagship by default. Both of these flagship lines are updated ~annually, with major upgrades happening ~every two years (iPhone 3G --> 4 --> 5 / Nexus One --> Galaxy Nexus). The in between years see step increases (3GS --> 4S / Nexus S). If you're a normal consumer, it makes sense to purchase one of the major upgrades, keep it two years, and then purchase the next major upgrade ad infinitum.
So basically, GN >>> 4S in terms of freshness/sexiness because the GN is a major upgrade while the 4S is a minor upgrade. Major usually implies design overhaul in some sense. If you're platform agnostic it simply makes no sense to tie yourself to the 4S for two years over the GN. The 4S will quickly be thoroughly outshined by the 5 whereas the GN will still feel quite fresh 21 months from now.
Just think of it like buying a new car as soon as its design is refreshed instead of 3 years into the product cycle.
Full disclosure: proud owner of both a launch day Nexus One and Galaxy Nexus.
