Wordpress - Opinions whether to go multisite VS 3 separate WP installation

You can use one installation to serve three or more single-site setups (just switch on $_SERVER['HTTP_HOST'] in your wp-config.php) or one multi-site installation. But running three updates for the same software on one host doesn’t make sense. Just a waste of time.

1. Unless you plan to install hundreds of different themes and plugins on each instance you can share the same plugin and theme directories on multi-site or different single-site-installations.

2. The database will not be a problem. And again you can reuse the same data base in multiple single-site installations too.

3. There are OpenID plugins to allow such a login. No relation to multi- or single-site.

4. There are no big differences. Some plugins work with less problems on single-site installations … and that’s why I recommend a multi-site: You will write your own plugins or themes sooner or later, and writing those for a more complex environment that you are actually using will make you a better developer.

Multisite is more useful in a scenario where the,

• context of all sites are shared amongst a common purpose and topic
• even when that purpose might be to allow people to run blogs on rAnDoM topics.

That (randomness) in itself is still the common goal and because of which you would tailor your themes, plugins and everything else to follow suit when catering for your users.

Notice that I mentioned the terms "your users" because...

• Its a you and them scenario.

This is where Multisite excels, in being able to provide you with the means and methods to run a network of user blog sites and to control and communicate with your blog site owners.

That said, its perfectly acceptable to use Multisite as a sole user, but you'd have to question whether it makes sense too and that has a lot to do with your intended purposes surrounding your websites.

As an example, I run several niche websites each of which on a separate WordPress installation.

Some sites are hosted on the same servers, some on the same IPs, some on different, some on unique IPs and others on completely different web hosts from different countries.

This has its benefits in,

• portability
• self-containment
• security (to a degree)
• SEO (if applicable to your cause)

...to name but a few.

If I want to sell a site, which sometimes is the case, then that becomes easy too. Because standalone installations use,

• standalone databases

Mulisite installations use,

• one master database with each site having its own separate table within

There are plugin initiatives that allow you to utilize separate databases like this one,

• Multi-DB

...but by "separate" they mean you can either have,

1. 16 databases
   
   OR
   
   
2. 256 databases
   
   OR
   
   
3. 4096 databases

For which your sites get evenly distributed amongst. So while they are separate databases, they are not separate in the individualistic sense.

There are probably other attempts at using single, standalone databases per site with a Multisite installation but it somewhat defeats the purpose... kind of.

Logistically it might seem harder to manage many-standalone sites and the more you have, the more involving your strategy sometimes needs to be but there are plenty of means and methods to help.

Depending on your purpose and business model, whether its for profit or please or a combination of the both - you will eventually develop your own tools and processes for quickly and easily managing your separate installations.

Its tempting to throw everything into one kit, one installation and consolidate it all BUT in doing so, what appears to be more efficient, could in fact bore more of a hindrance long term.

As @bryceadams said, installing and playing around with Multisite will give you a good feel for its capabilities and work flow. You'll better be able to determine its suitability to you there after.

Although...

My recommendation is with standalone installations.