and 10.100.100.129 255.255.255.192
both allow a range of 10.100.100.129-10.100.100.190 as per IPv4 Network Calculator http://www.klaban.torun.pl/prog/netcalc/
Why would I choose one over the other... also, the 128 subnet gives a bad mask error whereas the 129 does not... any ideas why?
.128 is the subnet ID - it holds the same concept of assigning an interface this IP address: 10.100.100.0 255.255.255.0 - Got it ? :)
Per your example, it will also fail if you assign the IP 10.100.100.191 255.255.255.192 since that's the broadcast address.
In short, you should never assign a host the subnet ID or broadcast address.