Both properties lead to the fish eventually suffocating so I think the importance of which is more toxic is moot.
Ammonia can and will severely damage the gills (lamellae and primary filaments) refered to as gill hyperplasia
preventing the fish from gaining O2
from the water and suffocating. The higher the level gets the less the fish will be able to excrete ammonia as waste. Nitrite oxidises hemoglobin into methemoglobin reducing the bloods ability to carry O2
. Again resulting in the fish suffocating.
If fish are left in low amounts for long periods, either can cause severe bacterial problems.
In short, both should be equally concerning to hobbyest. Of the two though, ammonia is typically the faster killer. It really depends on if your comparing equal quanities of both...