Either one will work... 16.9-34 would have been the more likely of the two tho. Best thing is to measure the wheel from inside bead to inside bead surface. If it's 15" or wider go with an 18.4. If it's a 13 or 14" wheel, go with a 16.9
I wouldn't get too excited about it harming the tractor having mismatched tires. That's what differentials are for... The bigger reason to change would be the fact that the tractor is not level.