One consideration for redeeming miles in business versus first is how many miles you have in your account, how fast you accrue them, and whether you want to save them for future trips. For most airlines, the difference in miles needed between the two classes is not that great. So unless you're really trying to conserve your miles, I'd say definitely go for first on CX if you can get it and probably go for first on UA if you can get it. In both cases, F is that much nicer, but especially for CX because it's really wonderful.

Ironically, F is sometimes easier to book for award tickets than business class is.

The only caveat here is that if you think you'll sleep most of the way then you won't enjoy the perks of F that much. On the other hand, if you're sleeping because you'd need to be fresh for business as soon as you arrive, then, the argument for F remains strong.