It doesn't make a lot of difference, as it's the charging circuit in the phone that determines how much current is applied to charging the battery. Usually, this doesn't exceed about 1,000 mAh.

So, either of the above chargers provides enough power to enable the N900 to charge as fast as it will. You won't harm it by connecting it to a charger that can deliver 1A, 2A, or 3A; the phone will draw what it needs, but you don't stand to gain any advantage by using a 1.3A charger over a 1.2A one.