The GN route will be quicker than the TL side as there is nothing to learn! Dead simple routes, no diversions etc, few yards and sidings. It's a train drivers dream really! The TL takes a lot longer as you have complicated core routes (probably more speeds on the Wimbledon loop than in the 78 miles from London and peterbourgh!) and you also have loads of diversions such as beckenham, crystal palace, Wallington, Lewes etc etc.
You can sign the GN in a week of assessments with a DM. Takes longer to sign everything on the TL.