For historical reasons, I have "Site A" on "Hosting Account A", and "Site B" on "Account B", totally independent accounts with the same vendor, Bluehost. Both are primary domains. Now that Hosting Account B is just about to expire, I'm considering letting it disappear and moving Site B to an Addon domain on "Account A".
Both sites are non-commercial, narrow-interest, very-low-traffic, hundreds of page views per month. The file weights for the sites are non-trivial, especially as I like to install specialized CMSs in subdomains. Since Bluehost allows unlimited hosting space there should be no issue with the file load, except I've seen hints of an issue with total file count, maybe 50k files -- which I'm not currently close to hitting, but might eventually.
My question: what are the pros and cons of using separate accounts versus hosting Site B as an addon domain?
Obviously, using a single account is cheaper by half, and I know that my authoring environment (DreamWeaver CS5) complains when it detects nested source trees, telling me "Synchronization" might fail in such cases, but I don't depend on this feature.
What other factors should I consider?
Added in response to PatomaS:
Thanks for your comments.
Site A and Site B are currently running custom-built php-driven code, no CMS and no DB associated with either. These sites do require some simple .htacess manipulations. A bit more work keeps those remappings from being applied to the subdomains, but that's under control.
When I install a CMS in a subdomain, I'm careful to use a completely independent DB for it. I just don't seem to have problems with CMS operation in subdomains.
SEO isn't really an issue: the primary domains contain totally noncommercial sites. In practice, I use CMS's in subdomain for non-public purposes, releasing the URL only to interested parties,
Switching over Site B to an Add-on Domain of Account A: I'd like any switchover to occur with a minimum gap, but with only a few visitors a month, it seems unlikely that even a 24-hour switchover downtime will be noticed by anyone. After that switchover, Site B should appear at its previous URL and operate exactly the same as before, so I'm not sure about what issues you anticipate.
Other than the htacess issue referenced above, and the problem Dreamweaver has handling nested site directories I mentioned in my original post, I'm not seeing anything really difficult internally. You say, "...there is going to be one site..." but as far as I can see, from the point of view of the outer world, Site B hosted in an Add-on Domain on Hosting Account A looks totally like it did before, when hosted in a separate account. What am I missing?