I would suggest you take a page from the XCOM series. Perma death can add more immersion than any ressurection method can ever achieve. If you have spent the past several missions to perfect the skills and such of Character 'Y' to perfection and suddenly that character dies, you will suffer an enormous feeling of loss. You made that character, you loved that character, but now it is gone, never to be seen again. Sense of loss is a truly powerful feeling, which should never be ignored.
Futhermore, perma death is in fact a feature quite a few gamers truly enjoy, because as mentioned it adds to the immersion, but also adds value to each individual character, which is often not present when characters just ressurect once a mission is over. Because, why worry if he'll be back up on his/her feet when all this is over? Of course, while quite a few players enjoy this aspect, others do not because they cannot cope with that sense of loss in a game. However, this choice of course should be made based on the audience you want to attract for your game.
If you want resurrection, sometimes it is better to just not explain. This time I refer to the gamer favourite Skyrim. If you die in Skyrim, it just reloads a previous saved game state, you just restart. No magic, no gods, no science, no explanation whatsoever. It is just a reset so the player can try again.
I hope this rambling helped you in some way!