we should come up with better arguments
Here is my modest suggestion. Naturalistic ethics. A rational agent has moral significance if it (or its coalition) engages in Nash bargaining with you (or your coalition). That is, you should make nice to it, only if it rewards its benefactors, punishes its malefactors, and gives strangers the benefit of the doubt. The amount of good you should do for your coalition members ought to balance (at the margin) the good they do for you. Your coalition works best on the basis of complete honesty.
There are no other moral principles beyond (long term) rational self interest.
One nice thing about this approach to morality is that it is sufficiently well defined to allow you to prove things. The other nice thing is that it does not require additional arguments and principles to
Somehow, I find it hard to imagine a principle or argument which would explain why I ought to be particularly nice to folks with lots of sentience - people who can receive more qualia per second than ordinary folks. However, being particularly nice to people who have the power to help or harm me and my friends - well, that is just common sense.
How do you choose who's self-interest to act in?
If two people are acting rationally, and they have the same information, they should come to the same conclusion. As such, it would seem reasonable that they'd all pick the same self-interest to act in.
It has been suggested that animals have less subjective experience than people. For example, it would be possible to have an animal that counts as half a human for the purposes of morality. This is an argument as to why that may be the case.
If you're moving away from Earth at 87% of the speed of light, time dilation would make it look like time on Earth is passing half as fast. From your point of reference, everyone will live twice as long. This obviously won't change the number of life years they live. You can't double the amount of good in the world just by moving at 87% the speed of light. It's possible that there's just a preferred point of reference, and everything is based on people's speed relative to that, but I doubt it.
No consider if their brains were slowed down a different way. Suppose you uploaded someone, and made the simulation run at half speed. Would they experience a life twice as long? This seems to be just slowing it down a different way. I doubt it would change the total amount experienced.
If that's true, it means that sentience isn't something you either have or don't have. There can be varying amounts of it. Also, someone whose brain has been slowed down would be less intelligent by most measures, so this is some evidence that subjective experience correlates with intelligence.
Edit: replaced "sentience" with the more accurate "subjective experience".