Have you hung out in the forums for both carts? They each have their own pros and cons. IMHO, the biggest issue with Zencart is that it's getting old - v1.3.8a has been out for a long time, and the look and feel of the basic cart is getting very dated. IMO, it is in danger of becoming like OScommerce - useless without a list of add-on modules that then make upgrades a nightmare. Of course you can use a heavily customized template, but that often means you need to get your hands in the php. I haven't put Prestacart into a production site, but I have been testing it. The base cart is a bit more modern (nice handling of images, etc), but it has a huge issue for me - the lack of good USA shipping modules (USPS/UPS/Fedex) and USA payment processors. Since you are in the UK, that might be a non-issue for you, but for me it's a show-stopper. I'm also not a big fan of Smarty Templates - I spent several years in Xcart battling Smarty and I'm not anxious to go back...
I am recommending PrestaShop. I moved my shops from ZenCart to Presta.
* Easy to use - as store admin and costumer side
* Looks modern
* PDF invoices
and many other good things.
ZenCart has more possibilities right now, but PrestaShop is also good background and with each release its going better.
I love zen cart, a bit of a learning curve, bbut great. The fact that it is old is why I looked at other products first. I came back to zen cart when I was not happy with other ones I looked at. It may be old, but new mods are still created every month and old ones are updated.
My only regret was not removing mods that I did not end up using, so some of the code is sloppy, but it does not affect performance.