Tough decision. The performance difference between an ultra-low voltage i5 and i7 shouldn't be that much, as both are just dual-core processors. Yes, the i7 has 1mb more cache, but from my last check two years ago (for an Zenbook UX303LN btw) the opinion was that it's not really important for most applications.

There was also a problem with the QHD displays back then, as the color fastness wasn't right. The yellow was more like a mustard brown. The reason for that was the new technology of the QHD display panels and it wasn't really fixable. For that reason many people were interested to get the FHD-version. If you want to use the system for photo editing, video editing or something else with correct color settings, I highly recommend to check if the panel problem still exist.

120€ for an upgrade from 128GB to 512 GB for a M2 SSD is very cheap here in germany, and believe me, 128GB isn't that much today. Also the lighter weight and the better connectivity of the UX330 (two USB3.0) are of course very tempting. On the other hand, the option to upgrade the RAM up to 24GB is also very important for a long use of the system. About the battery life I would try to find some review with tests.

Maybe some questions to you: Are you willing to upgrade the system by yourself in the future? How expansive is it for you to get a M.2 512GB or M.2 256GB SSD? If it's near the 120€ price range, I would go with the cheaper UX310 because of the option to upgrade the RAM and SSD, but also beacause of the dedicated GPU. Yes, you maybe don't want to game on that system, but maybe someday in the future you're happy to have the option to do so. Also you can use the GPU for other stuff, as example gpu-accelereated computing (https://en.wikipedia.org/wiki/CUDA).

Btw, my plan back then was to get the cheaper UX303LN with FHD display, i5 and 128 GB SSD. Then I wanted to upgrade the RAM and SSD later by myself. Unfortunately there was no chance to get any Zenbook here in germany, first the UX32LN and then UX303LN. Both were never in stock. After maybe six months of waiting and many mails between me, Asus and the shop I preordered every month, the release was suspended just for another month again. I finally gave up on the Zenbook and took the chance to buy a Macbook Pro 13 Retina relatively cheap.