It is like comparing Keane and Scholes, I know they were at a much higher level than Garner and McTominay but Keane and Scholes were as much different to each other as Garner and McTominay are. Another one, Fletcher and Carrick.
Garner and McTominay are not the same type of midfielder. Matic is on his last legs, Pogba is up in the air to stay, Fred is 28 (which by the time Garner is ready, Fred will be the 30 and will need replacing at some point), VDB won't be here much longer imo and doesn't play CM really anyway. Logically we can't replace Matic, Pogba and Fred without having to buy in other areas as well, so Garner and McTominay will most likely both be in the squad together.