I go back to original design here. Was the game intended by it's designer to have really stiff flipper buttons?
Go to factory original and you won't go wrong.
I've often wanted to replace the flipper buttons on Capcom's Breakshot... very stiff springs in those!
However, in your home, you should do what makes your game fun for you.
I'd also say that different flipper systems have a radically different feel. The flippers on a Data East 90's machine are poweful, but you can't 'feather touch' or play with the flippers to get control of the ball the way you can with a perfectly adjusted set of flippers on an Addams Family. It's not just the buttons (though the button springs that I remember are stiff...), it's the whole flipper system.
Flippers are the most important part of your game. My friend James told me "If you can put a quarter in it, get a ball onto the playfield and hit it with your flippers, every other thing about your game can be broken, and you'll still make money." Hehe.
I actually like a tiny bit more stiffness to the flipper buttons on some of the 90's WPC games. Using just the plastic flipper opto interrupt feels weak. I'll generally put in some spring steel to reinforce the plastic:
