iPhone 4s trade-in value / worth it to wait for 6?

What is the consensus on the trade-in value of a 16 GB iPhone 4s and how long it will be worth anything? Right now retailers like Best Buy and even Apple are still giving folks $150-200 for a used 4s. My Verizon contract is up in August, at which point I will be looking to sell / trade-in my 4s. I'm wondering though if I should do it immediately, or if this 4s will still be worth anything if I wait for the iPhone 6's to come out later this summer (figuring no later than October?) If this 4s isn't going to be worth much in terms of trade-in once the 6's come out, I will probably just get a 5s in August. But if it might still be worth something, I may hold out for a 6. Curious as to any opinions / advice? Thanks.