ATP Ride
The ATP ride is the same, the difference is which written you take. You do not have to take an additional written exam ever. If you have current 135 experience, it may be in your best interest to take the 135 written. If you have 121 experience, it may be in your best interest to take the 121 written. I took the 121 written without any 121 experience. I do, and did prior, have 135 experience, and my last 135.293, 135.297, 135.299 ride counted as the ATP ride because the written was completed and I had filled out an 8710 (look at the ATP regs under Part 61) even though the written I took was the 121 ATP written. My certificate just says ATP Multi Engine Land.
The ride is not easy, but not insurmountable, either. People do fail the checkride. I did mine in a Navajo, but you can do it in anything. There is an ATP single engine land, ATP multi engine land, ATP single engine sea, and ATP multi engine sea.