I own and have used both rounds for deer. The 7 mag is over powered for this use but will put big deer DRT at ranges up to 500 yards IF THE SHOOTER IS QUALIFIED but ruins a lot of meat in doing so. The 7x57 with factory loads is a 200 yard deer rifle since those loads are way under loaded in deference to the many old Mausers still being used. The performance of this ammo is uninspiring to say it nicely but it provides a useable deer load at a reduced recoil level. In my Interarms Mk X and custom M98, I load the 7x57 to slightly above 7mm08 levels making it a true 300+ yard deer rifle.
We killed several deer with both these 7x57 loads last year so have recent real life experience with the performance.