Gardaland has 4 OK coasters (Oblivion, Raptor, Blue Tornado, Mammut), Mirabilandia has 3 awesome coasters (iSpeed, Katun, Divertical). I think that Gardaland's coasters can't really compare to those in Mirabilandia really. They are great themed but that's it, except maybe Oblivion where the first drop is amazing but that's it, there isn't much of a ride after the drop. iSpeed and Katun are fast, forcefull, fun and all the way great coasters, and Divertical is one hell of a fun ride. Both parks have also smaller coasters that aren't much really. Gardaland has a great themed Jungle Rapids ride (Mirtabilandia also has one but not that great themed) and an awesome underground Disney style boat ride. You can have much more fun in Mirabilandia during hot days because you are soaking wet on water attraction whereas in Gardaland you don't get wet really and there isn't a lot of trees you can hide and chill under.
If you are looking for adrenaline and fun I would definitely suggest Mirabilandia!