What web platform to use for a $5k site - Wordpress or Joomla!?

Hey guys,

I started a $600/mo seo gig for a local client last month and at our first meeting suggested he get a custom website design. Well, this week he called and agreed to a $5k website deal starting next week. This is the biggest website job I've landed so far and I feel like I'm in a bit over my head. I really want knock this website out of the park, however I build it, and I've estimated 4-5 weeks for completion.

The thing is, I'm used to creating all sites, both for myself and small business clients, in Wordpress, but I'm wondering if I need to go further than Wordpress for this one. I don't have any programming knowledge so I'd really like to use a platform to work on, instead of building from the ground up. I will probably outsource a little design work but I'd really like to teach myself along the way where possible.

To give you an idea of the kind of site I want to build, check these two out:

installautomation dot net
finitesolutions dot co dot uk

It needs to be pretty slick with bells and whistles and I want to avoid using flash if I can use a slider widget for the main images instead. The site will also need to include an ecommerce shop.

What platform do you think would be most appropriate for building this site?

Also, what do you guys think of using Yoothemes for Wordpress/Joomla? Would their "Zoo" content builder be good for including an ecommerce shop on the site? What's your general opinion of Yoothemes and their tools?

Would really appreciate your advice and recommendations on what to use.

Thanks