GRINDCORE IS THE GREATEST GENRE OF ALL TIME, ALL OTHERS ARE POSER GENRES

Thrash is limited in its inability to go faster than the defining thrash beat before it becomes associated with the stylings of death metal. Grindcore has it's slow parts but changes quickly before it drags on and becomes something other than it's genre. For me death metal reigns supreme it's always evolving and has the most room to expand without changing its musical form.

Thrash is so limited and rehashed it's never evolved anywhere beyond cleaner recordings with current technology and it's blue print remains the same. But grind core is the height of all out energy no other genre can capture with such ferocity. To be dynamically uppercut out of your boots and outright obliterated by the desperation and hostility performed by the bands is a real rush, it feels like it could fall apart at any second but the good bands pull it off and it's a bloody rewarding relief when you survive the onslaught, but the onset of adrenalin leaves you starving for more.

Grind must be experienced live before written off completely and then you'll see it makes a lot of metal bands look like stunned statues playing to a lifeless click track.
 
Very much, I too find thrash rather limited and it has always struck me as a sort of 'transitional' genre and I can kind of see the point of those who say the genre is basically dead and that the newer bands are mostly rehashing the sound of the prominent thrash bands of old. Not to say that there isn't good stuff to be found within thrash, of course, but compared to other richer and more diverse genres such as death, doom and black metal, thrash seems rather stale.

I'd still take thrash metal over grindcore tho.