Subdomain or subfolder
It depends. You say "Visitors end up on those landing pages by clicking a Google ad."
By this, do you mean that you users will only reach those pages by clicking on ads? And that you then don't really need these pages indexed? Because if that is the case you can of course
noindex the pages and put them on whichever subdomain you like.
If the landing pages would also be useful to a user clicking through from another site or organic results, then it's generally better to use a subfolder. See the answer linked to in Stephen's comment.
Tracking in Google Analytics
Assuming you do use a different subdomain, a visitor will land on c.domain.com and may click through to www.domain.com. When they do this, their session on c.domain.com will end, and a new one will begin on the www.domain.com property. Setting them up as different properties won't fix this problem.
Instead, set up cross-domain tracking. This means the same tracking code is used across the different subdomains. There are a few different ways to set that up—see the official documentation here—but if in doubt about how to proceed I'd strongly recommend using Google Tag Manager (described in detail on that page).
Once you've done that, all the hits on the subdomain and the main domain will belong to the same GA property. So you need to watch out for page paths.
Google Analytics only reports page paths by default, so when you look in the landing pages report you won't see c.domain.com/some-page listed there, but just /some-page. There's no way to distinguish whether the page was on c.domain.com or www.domain.com. You can get around this in one of two ways:
- Give your landing pages on c.domain.com distinctive page paths. For example, don't have a
/contact page on the subdomain and on the main domain, because this will make your reports difficult to interpret.
- Get domain names to appear in your reports, by consulting this section of the documentation.