one app - multiple instances - how to maintain?

Hi,

I've created basic cms/ecommerce application. I've got a couple of
clients that are going to use it. I'd like to find a way to maintain
the same code base in the back end, but customize the front.

There are a couple different schemes I've thought of that could solve
the problem.

rails engines with svn externals
my own plugin scheme that kind of does what engines does, but more
simple. uses svn external
subversion branches?
KISS - keep all of it separate - manually keep track of bug fixes,
updates etc.. maybe use svn diff?

Surely a lot of you have created apps like this, where you want to
have different instantiations, be able to customize, but have the
ability to maintain easily?

I'm all ears :slight_smile: