mANT 19s product model is RB921GS-5HPacD-19S, see
http://wiki.mikrotik.com/wiki/Manual:Product_Naming to understand product naming conventions.
I haven't actually used it, but seems to be based on the same platform as netmetal 5 (RB922UAGS-5HPacD-NM): same CPU, RAM, etc... with the only differences being not having an USB port, or a minipci-slot for a second potential wireless card.
Performance is relative to your stations distance, signal, SNR, CCQ etc... statistical distribution, but think I could say performance from the AP radio/CPU point of view should be similar, which in the case of the netmetal, is the most powerful in terms of capacity in the routerboard lineup.
How many clients do you plan to serve with one mANT19s?