Maybe it is because your 5 is lighter, and also out in more wind, so it works well up-looping as it still accelerates up.
Whereas the 8 is heavier and in less wind at its low end, so when flown up it might lose speed and power, thus not as efficient.
Could be that, OR, just you who has a preference.
As there are so many parameters which interfere.
I prefer the up-loop, in particular if marginal wind with big kites.
But I also jibe first and switch feet going out of the jibe.
Where others switch feet first.
When you downloop you will have the kite at high speed very low going out of the jibe, which is horrible for switching feet, and you also lose upwind ground a lot more than with up-loops.
But if you switch feet first, you would probably prefer the downloop.
I like, having light kites, that they generate power when flown up, and coming out on the new tack kite is up and flying down, so here you can dose how much power you want yourself, but more importantly you got a kite sitting higher which makes foot switching a hell of a lot easier and safer, and after this you still have a kite that flies down so accelerate you so you can keep foiling without losing upwind as much.
You can also hold the lines tighter easier with the up-loop, as the angle when kite is higher, means tighter lines around in the carve.
When downlooping you have to sync so the kite does not downloop too early, as then you will get slack lines a lot easier when carving, more risky.
But all'n all I still think it is personal what one prefer, depending on many variables, so different for all of us, and sometimes different when we change size, as you have experienced knyfe