Thanks for the detailed answer and the hints
I guess for a casual-gamer skyblock is then better because you pretty much get free ressources through sieving.
Or does it take away alot of challange when it comes to search for alternatives to mining (like bees, these infused thaumcraft flowers - only read about them once - and such things)?
It depends what your play style is. If you don't like mining too much, then either one is just as good.
However, if you like mining, go for the non-Skyblock.
However, if you go for the skyblock, I recommend you get an auto-clicker, Murgee's Auto-mouse click program, is the best I have found so far. Reason being is you will find the early game quite boring and repetitive. And using these types of software you can set up a "mouse macro" (murgee allows for keys too) to just hold down left click with the chicken hammer to get yourself a load of gravel.
Then to scroll up, right click (place gravel/sand), press R (keep the stack full so the process continues), scroll down, left click (break block)
And then one which will right click a few times, and press R (for the sieve)
And then you can run them when you go to get a drink, make some food, go for a shower/bath, etc. Just so you are ignoring some tedium; I just sieved 20 stacks of dust while in the bath using this. Now I can finally get a cobble stone generator