HMVC Vs MVC over subdomains.

I am building a largeish well complicated application with Zend Framework or Ruby On Rails now i've found the HMVC model makes managing this application pretty much childs play though it is limited to Zend Framework.

My question relates to if it would be more efficient to instead of making each of the component applications(forum, gallery, blog, stories, account, shop) into modules of one large application or if it is better to separate these into sub domains using MVC per component application?

I am expecting a load of between 1000 and 10,000 maximum registered users with perhaps no more than 500-1000 at the same time.

Are you planning to custom write each of these aspects? Why not piece together multiple open-source projects? That would save you so much time and security testing.

If you need each component to have the same look, then theme-ing is much simpler than writing that many components.

I am planning on custom writing most of the components or perhaps all of them as this is a personal project i have all the time in the world to enjoy the programming.

Individually non of the components are new to me when it comes creating them, i've written a forum before etc.

Mostly my concern is if i should have the application split over subdomains like forum.mysite.com, auth.mysite.com, blog.mysite.com, users.mysite.com, stories.mysite.com with each having there own MVC and using a shared database and some form of communication between modules. Or if i used Zend Frameworks HMVC i can set up a separate MVC for each module and a shared model base as one monolithic application.

Other things the application is going to have are a desktop based admin which i am currently building in Objective C with a Objective C MySQL class which i have been able to get together very rapidly in Xcode.
The modules of the application need a shared login and if a module or modules get heavy traffic i would like to be able to quickly move these to another server.

I've not worked on a project with "all the time in the world" since I was in high school. Personally, given that time frame...I'd make each component separate yet with ability to access the same database. You could easily do that with a simple header PHP file to connect with the MySQL database.