Well, if you got like 5% of donations per task you did I would definitivly choose that. If you look at the economical aspect.
You do 4 tasks on 10 server (20% on each). 2 servers earn zero (bad projects that never got players)- The 6 servers each earn 150EUR, but the 9th earn 600EUR in total, and the last one earn in 1000EUR in total. Thats 2500EUR, and you get 500EUR. And the projects might go on for a longer while too, giving you higher potensials then that.
But still, it depends on the work amount and how much they are willing to give you in %'s. If it's really hard work you should get paid for that task alone. If it's 5-10min work, try to get %'s.