If you want an iPhone, it's game over there, since you get what Apple offers today.

1) A 4G phone has 3G failback in case of an 4G issue.
2) For a while, there may be less 4G users than 3G users in an area. As time goes by, this may reverse, everyone is upgraded to 4G and fewer use 3G.
3) In theory, a faster network, albeit shared, allows more users to consume the same amount of bandwidth then a slower one. A 50K web page is downloaded in less time, and that user isn't using the network anymore. The problem comes from increased demand, when everyone who was streaming 320x240 youtube videos starts streaming 720x480, etc.
When some sites autodetect what your client can do, they automatically upgrade you into the higher bandwidth product.

Other than that, buy the iPhone, enjoy it for a few years, then when the [internal] battery life starts getting problematic, either have Apple replace it or upgrade to the current model.

The other consideration for 'speed' these days is the phone's processor(s). Anecdotally, the faster phone on the same network seems to be faster. Again, not an issue if you have committed to an iPhone, but more of an issue with selecting an Android phone.