Recs.
Updated
SpecsUpdate
Pros
Pro Bright screen makes it easy to read even in daylight
The 5.5" 1080p IPS LCD display provides up to 583 cd/m² of brightness, which is easily bright enough to see even in direct sunlight. This is very important because while playing Pokémon GO most of the time you'll be outside during the day looking at your phone.
Pro Good battery life
While the battery may be smaller than Android phones, iOS is very efficient and makes the 2750mAh battery last for 15h51m of continuous 3G calls, 13h57m of back-to-back video playback, and 11h51m of constant web browsing. This shows that the display isn't a battery hog, which will lead to good battery life when playing Pokémon GO.
In a battery torture test, the iPhone 6S Plus ran out of battery after 3h17m on full brightness at full load. You can expect at least this much time when playing Pokémon GO, and in tests it hasted 4h41m.
Cons
Con Large form factor makes it easier to drop
The iPhone 6S Plus is taller, heavier, and also a hair wider than other 5.5" phones. This makes it more awkward to hold as it's less balanced in your grip, increasing the chance of dropping it if you are caught off guard by a car or person while trying to catch that Pidgey.