The effects are bit similar, but the mechanics are different. The Tradition opener reduces the rate of increase in the culture cost of acquiring tiles, while Religious Settlements directly reduces the culture cost of tile acquisition.
The base formula for the culture cost of the next tile is 15 + (10*tiles already obtained)^1.1, rounded down the nearest multiple of 5. The Tradition opener changes that to 15 + (10*tiles already obtained)^0.85 (also rounded down). This discount starts at about 33%, rising to above 60% by the time you have acquired 10 tiles or so.
Religious Settlement just reduces the cost of tile acquisition by a flat 15% (also rounded down to the nearest multiple of 5). Religious Settlements operates similarly to the culture cost piece of Angor Wat (reduces both culture and gold cost of tile acquisition by 25%) and the Russian Krepost (also -25%).
So, for example, the base culture cost of your 3rd tile is 40 culture, but Religious Settlements reduces that to 34, which is then rounded down to 30, and the base culture cost of the 4th tile is 55, but Religious Settlements reduces that to 45. By comparison, the Tradition opener would reduce the culture cost of the 3rd tile to 25 and the cost of the 4th tile to 30. So, Religious Settlements is beneficial, but no match for the Tradition opener.
