I would personally use Prestashop for your situations as I think it does variations better than Zencart.
I just looked at Prestashop which looked good, until I got to the variations (or attributes as its know in Prestashop).
It looks to me that I have to put each product variation in individually just like OS-Commerce, and for the amount of variations I need to do its gonna take me ages.
I was looking for something similar to the plug-in I was using on wordpress where when I came to add a products variations, I selected the radio buttons next to which separate sizes and separate colours that were available and WP e-Commerce calculated all the possible variations for me, instead of me having to do them one by one.
I tried to look for a screenshot of it but I cant find one and I don't have it installed anymore.