Depends on the Magnaguard. Fresh Magnaguards right off the shelf, the guard loses against two in a hard fight. Grievous trained Magnaguard that proved challenges to and kill Jedi stomp the Praetorian Guard and can arguably solo.
Seems right, tbh not sure the weaponry the Guard has is a good match for the Magnaguard, they have some pretty great durability with being able to survive headless. They also seem far more agile.
Magnaguards were more impressive, yeah. Obi decapitates one and it keeps twirling and attacking him, they'd probably shit on the Praetorian Guard, actually.
A single Praetorian is probably better than a single MagnaGuard, honestly.
The official G+ Star Wars page(run by LucasArts/Disney) notes Hux took into account Jedi training when training the FO Stormtroopers in melee combat, and that the Troops in the FO are trained far better than the troops during the Empire's day. Finn being in the top 1% of that speaks highly of his own skill(which I wouldn't hesitate to put on par or ahead of a standard MagnaGuard).
Finn wasn't capable of providing half the challenge to an injured Kylo Ren as a Praetorian Guard provided to a healthy, mentally stable one. 8 Praetorians vs 2 MagnaGuards is an absolute slaughter in the Guards' favour.