The value of your phone will decrease over time. There's not many phone that hold their value, maybe high-end iPhones, the SGS2, the One X would, I imagine, hold its value very well.
I am currently mulling over whether I should get rid of my Dell Streak and HD2. I have an SGS2 already and want the SG3 or One X. The thing is, I use the monstrous Streak as a GPS device bolted to my bike handlebars, and the HD2 as a WiFi modem for it to connect to the net. The SGS2 has only just now (after having it for a year) become my main phone. I love my HD2's ability to run Android and a result of that meant my SGS2 was basically a back pocket 'fad' device...until now.
Anyway, if you want full value for your phone then you should sell now. Phone prices can go up when an insurer or network need some but again that's more like BB, iPhone or something that's not SO old.
I am currently reviewing the differences between SellMyMobile and OnBay Recycle to see which comparison site will give me a good deal.