Although as it has been pointed out it is more of a personal choice, if you are trying to recreate the feeling of games from the mid 80's and onwards, I would recommend a 2 coin slot mech system. Also, as Peale pointed out, if one of the mechs dies, you have the other as a backup.
Personally, I like the 2 coin door look and feel. Also, single coin systems remind me too much of gumball machines for some odd reason... (I am sure some of the old school purists will hate me for saying that, as the really early games usually only used one coin slot)