I think you may find it helpful. What we usually do:

- register multiple domains in our hosts file (or router), customer1, customer2, etc.

- all these domains point to 127.0.0.1 (or basically to your local IIS)

- then we usually create separate start page types (or most probably templates as well). you can majority of properties probably extract to abstract startpage type class that will be shared across customers. Let's say you may end up with: 

abstract class BaseStartPage
{
    // shared properties like Heading, etc
}

class Customer1StartPage : BaseStartPage {}
class Customer2StartPage : BaseStartPage {}

- create also new startpage templates (usually to get new look and feel if needed)

- then we create multiple sites inside EPiServer: one for Customer1 and another for Customer2

- point each site to particular domain (registered earlier)

- and set start page for each site to particular start pages (probably you will need to create them first)

Now you are able to browse to customer site #1 and customer site #2 at the same time.

Btw, if look and feel is almost the same with some small exceptions, I would look for a way to create a property on the page that may regulate which CSS style file you pick. And again this custom CSS file may override only absolut minimum required for other site to look as required by that customer.

What are the main difference between these models?

You can differentiate through different page types and use base classes as Valdis suggested.

Other things you can do is to use Display Channels if you only want to display things differently depending on each website. http://world.episerver.com/documentation/items/developers-guide/episerver-cms/7/content/display-channels/

With this Display Channel you can resolve which css you'd like to use as well as using the same page type but different templates using Tags.

But again, what are the main differences between the models, for example start page and article?

1. I edited my host files as follows:

127.0.0.1 localhost
127.0.0.1 testsite

2. Went into Admin mode on the first site and created a new start page.

3. Went into Admin mode >> Config  >> Manage Websites   and did the Add New Site.

http:// testsite  and  gave it the start page that i created in #2.

4.  Tried to navigate to the nwely created site http://testsite   but it does not got to the new start page.  It's like the new site doesn't actually exist in IIS Express.

Recall that I am running in Visual Studio 2017 debug mode, which is IIS Express.  When I look at the taskbar for IIS Express, under the View Sites, I only see my first site (localhost:xxx1). I don't see testsite.  

Has anyone gotten thi running in Visual Studio Debug mode (IIS Express)?  This should be easily reproducible.  Just create a new Epi CMS project, run it with the sample alloy site, and try to create a nwe second site (multi site).

1. I edited my host files as follows:

127.0.0.1 localhost
127.0.0.1 testsite

2. Went into Admin mode on the first site and created a new start page.

3. Went into Admin mode >> Config  >> Manage Websites   and did the Add New Site.

http:// testsite  and  gave it the start page that i created in #2.

4.  Tried to navigate to the nwely created site http://testsite   but it does not got to the new start page.  It's like the new site doesn't actually exist in IIS Express.

Recall that I am running in Visual Studio 2017 debug mode, which is IIS Express.  When I look at the taskbar for IIS Express, under the View Sites, I only see my first site (localhost:xxx1). I don't see testsite.  

Has anyone gotten thi running in Visual Studio Debug mode (IIS Express)?  This should be easily reproducible.  Just create a new Epi CMS project, run it with the sample alloy site, and try to create a nwe second site (multi site).

For IIS Express you need to edit .vs\YourProject\config\applicationhost.config and add the bindings (search for: <site name=").

Some find it easier to use the real IIS but I always try hard to first get IIS Express to work as needed.

To avoid problems later on we always run everything on HTTPS as well.