A modern canned beer should never taste like metal. If it does, you're probably drinking straight from the can, and while the folks at The Alchemist might recommend that, I can't say I share their view.
Modern beer cans are lined with a water-based chemical that essentially ensures that your beer never touches metal. This in turn means that strictly speaking, canned beers will keep longer without being skunkedskunked, and pressurized packing means there's less air in a can then a bottle, which adds further preservative benefits.
That said, very few beers that are good enough for this decision to matter are offered in a choice of containers - most small breweries just don't have the capacity to both can and bottle - especially of the sort of small-batch stuff that you'd actually be interested in trying to cellar and age. Furthermore, there are significant environmental and economic arguments in favor of both bottles and cans.
At the end of the day, choose the packaging that's more convenient for you to store, consume from, and recycle when you're done. The quality difference is essentially insignificant.