How can I pick the correct replacement usb charger for my phone? My phone has a wall charger with outputs 5vdc and 850mA, which just broke (I saved the USB-microB cable which I've been using to charge from laptops). Unfortunately, OEM chargers of that particular model are not readily available in our area. So, I have to resort to generic/replacement chargers or adapters. Now, when I got to an electronics store, they don't have a charger with the exact specs. It's either a 700mA or an iPhone adapter that has 1A output current. Which should be somehow better/safe for my phone which has Li-Ion 1320 mAh battery? I've seen other threads which suggest getting higher outputs while some say otherwise.
Using a higher current charger will not be a problem.  The phone "pulls" current, rather than having the current "pushed" to it by the charger, so it will only take as much current as it wants, regardless of how much the charger is capable of supplying.

By relation, there's no point in getting a charger with a current rating higher than your phone will accept.  It's just a waste of money, as your phone won't make use of the extra capability.  Exactly how much current your phone will accept is usually unspecified and is generally only determinable by direct measurement (i.e. put a multimeter in the circuit), and thus generally isn't practical to exactly determine for an everyday user.

1A chargers are fairly standard and would be completely adequate for almost any phone.  Chargers with more current than that are generally intended for tablets/phablets which have much larger batteries and will accept more current (e.g. 10" tablets generally charge at around 2.5A).  Your phone will charge off those fine, but as I said above, just won't take advantage of the extra current capability you paid for.