There is no difference in the two commands, they both achieve the same thing. I believe it was changed to no ip domain lookup in a later version of the IOS so the no ip domain-lookup is an old version of the command.
because there are more options now. Both exist.
domain domain-list domain-lookup domain-name
Uber-Geek-R1(config)#ip domain ?
list Domain name to complete unqualified host names
lookup Enable IP Domain Name System hostname translation
multicast Define the domain name for multicast address lookups
name Define the default domain name
retry Specify times to retry sending a DNS query
round-robin Round-robin multiple IP addresses in cache
timeout Specify timeout waiting for response to a DNS query
Uber-Geek-R1(config)#ip domain look ?
nsap Enable IP DNS queries for CLNS NSAP addresses
source-interface Specify source interface for DNS resolver