Shopp 1.3.3 seems to be the most complete and stable release so far with 39 bugfixes and 61 closed issues in total.
See the announcement of Shopp 1.3.3.
My personal favourites among the fixes are that the order exporting is now fixed and the image quality and sharpness setting issues is resolved.
If you need help upgrading Shopp to the latest version you’re welcome to get my upgrade offering. This can save you a lot of time if things go wrong. Especially if you’re upgrading from an older version of Shopp where you will need to manually edit the shopp template files to take advantage of the new features in Shopp or sometimes even to get it to work at all.