It's always better to do well in a poor scaling subject that completely screw up in a subject that scales well.
Scaling is only your friend if you hit the state average, so, as a general rule, if get 49% in a good scaling subject when the state average is 50%, you'll do worse than your poor scaling subject where you get 51% if the state average is 50% (actually, with marks that close, it's probably not a big difference, but you get the idea).
Based on that, I'd do the subject that I'm enjoying more, because I'd be more inclined to put in the hard work it needs to get a good mark.
As for cosmology, I'm not doing it myself, but one of my good friends is. She new pretty much nothing about it when she picked it up, and is doing pretty well at it. But she says that even though it's a science, it's a lot more like English, because it's all essays and crap, even though all the facts and stuff are there for you.
To be able to do a distinction course, you have to be really motivated, because there's so much independent work involved. And lots of people find that difficult, because if you get stuck, there's not an easily accessable teacher, or friends who are doing the same subject, to ask questions. There's also not anyone constantly pushing you to get your work done, like lots of teachers do.
