I really enjoyed reading this thread! I'm researching carts for a client and this one is in my testing bin right now. I've been reading review after review on various carts and shortlisted things down to the following ...
Ecwid (on either Wordpress or Joomla) because that's what the client currently has and it works ... just needs an upgrade and a tweek
Magento (because the reviews all state it's well worth the time to implement regardless of the big resource requirement), definitely scalable for a growing business
Opencart (because I've seen several sites where it works well) but after this thread I may reconsider!
there were also several I really thought would be a good fit but I don't feel my client needs to pay a percentage of his sales to both the cart provider and payment gateway provider. There are too many other good solutions.
So aside from what I've already stated, perhaps I can get some feedback on what you would use for your client who has 5 or 6 custom made products, ships internationally, and gets atleast 25 orders a month.