Pro: Makes making easier Con: If the host company goes bankrupt or, far, far more likely the server fails, you are up the creek without a paddle because in one blow all your sites go off line. Having had this happen I am understandable a bit gung-ho. Actually I lost half my sites, not all, but now I'm using multiple hosts. Despite the extra work (and cost) it lets you see what host suits you best.
I tried hosting multiple sites in the same account, but I found it a bit confusing and cumbersome. So I got a reseller account and can now set up unlimited sites under my main account and they are totally separate. Works great for me. You might consider that option too.
I had a problem with Google results way back in the day when I added an additional domain on a standard cPanel account. I dunno what went wrong where, but the incoming links were really screwy and everything was properly setup. Just do a reseller account, it's easiest and is the right way to do it.
Reseller account is the best option, and you can still reach all of your sites from a central point. Having them all in one account just makes things complicated when trying to restore or move a single site. Also makes stats and some mod_rewrite interesting.
Advantages: Lower cost, easier to manage. Disadvantages: Google discounts or ignores cross linking sites from the same IP or class C range. They talk about it in their patent and I know they do it - at least in certain circumstances. The same IP may trigger a filter that checks the domain ownership to see if a site owner is "voting" for another site they own. There isn't any penalty, it is just ignored or discounted. If the host goes down, so do all your sites. I personally uses 6 hosts and keep a backup of sites on another host. If one host goes down for an extended period (it has happend more than once) I just change the DNS and the backup site starts getting traffic within an hour (although it can take 48 hours for a complete change). Multiple hosts allows you to crosslink sites. Right now google is counting these backlinks and allowing PR to pass. Hosting is so inexpensive that the benefit of cross linking outweighs any disadvantage.