@DeathToTheEmp: Thanks. And yeah, that's what I figured after looking at it a little harder: unless I'm strapped for points, or want a second power (taken with a Familiar), then the Daemon Prince seems to be the much better option.
@the Brayn: Interesting. How vital is it to get the squad of CSMs to 10 men, though, at 750 points? Is there any value in the idea of upgrading to an Aspiring Champion with a Power Fist, and taking fewer CSMs to pay the difference?
@broken: As for the Obliterators, I was thinking exactly the same thing as the Brayn. Having two of them means that I can select multiple targets if need be, and they won't be so easy to take out all at once with a template or single assault. Obviously it would be ideal to have more than one per unit in any size game, but with only 750 points, this seems to be the best compromise.
Re: Melta... I'm partial to that solution myself, though it means splitting weapon types in a single unit. In one of my variants, I gave one small CSM squad a Flamer and one a Melta, so they could focus on different roles. Giving them one of each makes them more adaptable, but less focussed. It's touch to know where to come down on that particular question. I think the best way might be to just playtest both.