IME Magnifica anemones do slightly better in captivity and respond better to Cipro treatment than Gigs. For that reason I'd always chose a magnifica over gigantea but at the same time never say no to a nice and healthy gig.
I think Gigantea would be better four you in a smaller tank. It is easier to keep the Gigantea just at the sand rock interface. Much harder to keep a Magnifica low down. He always want to climb up. A 37 gal you can keep a small Magnifica in 1/2 of the tank, and the other 1/2 keep SPD. Be careful to keep the Magnifica from touching the wall or other rock that is higher than the one he is on.
I don't target feed my anemones the guy at the LFS said these came from Tonga and there hardier than other Mag's from other places idk if he is BS. Also I wanted a magenta or purple base but none to be found