You maximize battery life on an iPhone by turning off WiFi and display, there are other tricks too. In that state, running Cyclemeter, I find my 4S battery will last 6 hours or a bit more. That isn't enough for a century, since you don't want to get stuck with 5% battery trying to make an emergency call. You can get external batteries to recharge the iPhone, anything from 1 charge from something the size of a pack of chewing gum to 5+ charges from something the size of two decks of cards. On long rides (100 miles) I carry one of those in the saddlebag.

What's the battery life of a Garmin with GPS? I don't know. EDIT: looks like it can be up to 10 hours if you don't use backlight or map display. Per posts I found.

Of course, the Garmin sits right on your handlebar and you can look at speed, distance, etc as you ride. The iPhone can be mounted on the bar, but having the display on will shorten battery life. There is a product coming, the Wahoo RFLKT, that is a bar-mounted remote display/control for an iPhone tucked safely in your jersey pocket.