Total production numbers, how many 6spd vs 9spd vehicles (RL) have been produced? Even if the 9spd has more units, does it have more miles driven? 6spd trucks will have higher mileage on average so the rate of failure should be higher.
It seems fair to say the 9spd is probably more durable and less issue prone. However if you have a 6spd, there is no reason to panic. Plenty of them are still driving around with no issues.
If for some reason the OP still needs advice, I would lean more to the 2020 RTL. Not because the 6spd is a deal breaker but because (to me) the difference between the 6 and 9spd is more significant than the difference between RTL and RTL-E.
It seems fair to say the 9spd is probably more durable and less issue prone. However if you have a 6spd, there is no reason to panic. Plenty of them are still driving around with no issues.
If for some reason the OP still needs advice, I would lean more to the 2020 RTL. Not because the 6spd is a deal breaker but because (to me) the difference between the 6 and 9spd is more significant than the difference between RTL and RTL-E.