I'm a big supporter of having a seperate installation for each site. Unless they are somehow related, why tie a site to another site? It just makes for headaches in the future if you do ever need to seperate them out. The biggest headache is on upgrades if you need to only upgrade one, but not the other, the multiportal solution can be a headache.

As for deploying to a host, it depends on your host, and what all you've done in your staging environment. If your host allows you to restore over a DB, or create a new DB, then yes, you can just copy the DNN folder and the database up. Just be sure to change the web.config

Not sure if I agree with such a blanket statement as seperate installs, as there's other factors to come into play.  For instance, multi-sites under one dnn installation allow you to share modules that you may use on more than one portal environment without the issue of licensing (on most) or installing updates on multiple dnn instances.

Also just the generalistic overhead of running under a NET session and related ascx caching, dll caching and thread overheard is shared then across all sites, and not per site.  Depending on your host environment, that could be a large advantage.  You'd also be sharing common dnn and other "host" task threads (ie: queue, etc) to create for a less memory intensive operation as a whole (as long as you have a good ISP that allows for tweaking of the memory recycle threshold).

Also, ease of administration - you can log onto one site as host, and perform administrative tasks for all your websites under management.

As far as difficulty in splitting them afterwards? not really, restore the complete backup of the db and files onto the other site, and delete the sites you dont' want from it.