My clients are also small local business owners and I'm building all of my clients websites with either Joomla or Wordpress. Don't even bother to look at the other CMS any more. Some of my clients had another CMS site like drupal or Typo3 and they didn't update the site because they were afraid to do something wrong. The system was too technical. Most business owners just want to run a business and don't want to study on how to use their website. I think both Joomla and WP are the most easy for them to use.
For me there are several factors to stay with J & WP:
The first and most important one is that there are 1000's of free extensions/plugins and theme's available.
If you build your sites for "dave the electrician", you can't afford to spent a lot of time building the site. From my experience they don't want to spent a small fortune on your skills, so you need a fast solution. I do the design myself and the rest is plugins, which are easily modified if needed.
Just like you I want my clients to be able to add text and photo's to their own sites. With both cms they can do so in the front end and they never have to look at the admin site if they don't want to.
I only have to give them a 3 page pdf with some images that show: click here, you'll see this, go there. Tell them where they can post the content and that's it.
I do offer my clients a maintenance contract and all of them agree on that. It's only to keep all software on the site updated to the latest versions.
Both CMS's can be updated very easily:
With Joomla you just have to upload the patches to your server, that's it. Most of the extensions are starting with update notifications so that part is also getting much quicker.
Wordpress is even easier: It provides a one click update option for WP and even all the plugins. (you can use plugincentral for this) So that's a maintenance contract for 5 minutes work. (I don't tell my clients this )
WP is much smaller and faster than Joomla, but both are very well documented and information can be found all over the web.
WP has excellent SEO options, with or without a plugin. Joomla is a bit more difficult, defenitely needs an extension, getting better with the new J1.7
As said before there are 1000's of plugins available for both CMS, but to my opinion it's easier to find the right Joomla extensions. (this page [WordPress › WordPress Plugins compared to this page: [url=http://extensions.joomla.org/extensions/]Home - Joomla! Extensions Directory](http://wordpress.org/extend/plugins/))
Your client wants a 20 page ecommerce store? Just install an extension and it's done.
For any cms you choose, try to look from your customers point of view. They need to update the site and don't have the technical background / website skills that most of us do have.
Anyway, just my 2 cents, I hope it makes any sense,