Well, I would go with Reality Warper under some circumstances:
A. Both of them are "equal" in strength.
B. Their first move will be their last.
Why? Well, what is reality but the present? Assuming that past no longer exist and the future is not yet here, reality is like the gary's mod god mode version with the user having the ability to alter the state of matter as they deem fit. In this case, the death of the Space-Time user is certain, solely because the Time aspect will be fucked and the Space aspect is severely hampered.
What is reality? But how we perceived the world.
Of course, The World fans will immediately jump up to defend that if the Reality Warper is stopped before they can use their ability, then everything is handy dandy. This point immediately makes the assumption A moot as now it is expected that the power level of ST is slightly, but undoubtably, stronger than RW as they are capable of ignore the RW's influence of the Present to give themselves an upper hand against RW.
Of course, the concept of Past to Future is always played with and assuming that the past versions exist, it is always possible for ST to just play the Girl who leaps through time and screw with the weaker RW. Again, this makes assumption A moot.
Then again... Space-Time being a subset to Reality can be argued with... case and point of Infinity War with the stones being Space, Time and Reality.
I just walked into an alley and got beaten shitless, if Time is what constitute in the stream we are in and it is through this river we perceive it as our reality, then it is safe to say that those who manipulates, aka travel, the stream is a form of reality warper. Yet if the reality warper warps their current surrounding to see as they deemed fit and the time stream is capable to alter the past to fit the present, while the RW is unable to travel through time, it is still capable of manipulate it via causes and correlations, making them shifters.