There isn't a station called Sunderland (Metro) as such. It just means that if you select 'Sunderland (Metro)' as the starting point the ticket is valid for commencing your journey from any station on the Nexus Metro system (e.g. Park Lane) and changing on to the national rail network at Sunderland station. If you just select 'Sunderland' it means your journey has to commence at Sunderland rail station (although you would still be able to start from a Metro station if you bought a separate Metro ticket from your starting point to Sunderland). Tickets starting from 'Sunderland (Metro)' are priced higher to make allowance for the additional journey. Buying one would make sense if your journey required a longer Metro trip, but from Mowbray Road it would be as easy to go directly to Sunderland rail station rather than walking to, say, Park Lane, and travelling one stop on the Metro - so just choose 'Sunderland' on the booking form.
Grand Central http://www.grandcentralrail.com/ is an alternative operator to East Coast trains for travel from Sunderland to London. Their trains don't go through Newcastle. As mentioned above they are less frequent but can be cheaper if you're booking in advance.