I'll go to Craigslist and see what the phones will go for.
The iPhone 4, I'll do $350
The iPhone 4S, I'll do $380 but I want $400
It's always good to sell early so I have the money before the new iPhone release becomes imminent publicly. I could get an iPod touch to hold me over until then.
Being one of the first people with the new iPhone is important, and I usually buy the unlocked version, which is way more expensive.
With any finances, there are some assumptions.
Sell now scenario:
-$650 (iphone 5)
+$400 (iphone 4S)
$250 (cost to you)
Take into account the price of buying/selling the iPod and the cost goes up
Sell when the iPhone 5 is released
-$650 (iPhone 5)
+$350(iPhone 4S) (using existing value of iPhone 4)
$300 (cost to you)
This doesn't take into account the cost of the iPod.
Up to a $50 difference for a phone that's coming in 3-4months...
Switch OSes, it always takes several months for apps to adjust to new phones, so the initial release will mostly give you bragging rights.
I own an iPhone 4 and my partner an iPhone 4S... They truly feel identical when using them. The iPhone 4 battery lasts longer.... No clue why, but not a big deal.