AFAIK the translucent blue is still available because Fairphone is confident that this model is not as faulty as the other colors. I don’t know if they changed anything about it though and I know that many people reported dissolving rims also with blue transparent covers.
So the durability point goes to the slim cover. Especially if you plan on proudly & regularly showing off how easy the FP can be taken apart for repairs. Even if the original covers were better than before now taking them off regularly will still be bad for them, while the slim cover’s mechanism seems to be built to last.
When it comes to protection I’d say they are the same - but that’s just a guess. I have the feeling that broken screens after drops are much rarer with Fairphones than most other phones and I guess that is simply because the cover(s) go over the rim of the screen - I have a feeling the material (elastic or hard) doesn’t matter.
I’m very happy with the slim case and can recommend it to anyone. If you want to read more reasons for that take a look at my review here.