You clearly haven't thought this through very well.
Plenty of people actually do need a new phone, that's (partially) why upgrades exist. It's not always out of convenience, or on a personal whim when we see a new model. We drop them, get them wet, kids screw them up- sometimes completely on accident, etc. <Looking at pile of dead/damaged Android handsets in corner...>
If you like/prefer the bigger size or aesthetics of a newer model, then go for it. Since I'm not a huge mobile gamer or movie watcher when I'm commuting, having the latest dual core processor, or one that's 1.8Ghz vs 1.5GHz won't mean much. In addition, don't forget that a phone with supposedly better or faster specs in real world can actually perform a little worse depending on the carrier ROM installed. That's why a lot of us modify the heck out of some of these devices, to get them to perform at full potential.
From your reply I'd say you hit the nail on the head regarding the economics of the situation. If you feel you have an antique in 6 mos, then definitely stay 'current', so you can sell for the highest price when you want to upgrade to something newer down the road. This should help you pick a handset more easily since you are not upgrading out of necessity.