For building muscle, neither is automatically better than the other. It varies on what you do with the movement. I have found SLDL’s easier to use for hamstring work and RDL’s for general deadlift assistance work. I don’t use either for mainly muscle building.
For strength I’d say RDL’s have more carryover to regular deads but once again what matters more is what you do with the movement. It also matters what your weak spots are. Adding a secondary squat session with front squats to better your regular back squats might help you more than a secondary back squat session if you have trouble with your quad strength for example. Same thing with SLDL’s and weak hamstrings.
Both and neither are safe depending on how you are able to perform them.
You should choose the one that answers your personal goals better. Nobody here can say which one is better for you with these specifications.
Elevated stance = you can go deeper with the bar = more range of motion = more muscle activation. But also a greater chance of injuring yourself especially with higher weights.
For deadlift strength, RDL’s from the floor.
For squat assistance, SLDL’s not touching the floor with the bar. Aim to work the hamstrings as much as you can.
For purely bodybuilding purposes, SLDL’s since they might be easier to learn and utilize in strictly hamstring and glute activation.