A little late to the party but I wanted to chime in on the slider debate. I have eaten at White Castle, White Manna (Hackensack) and White Rose (Highland Park).
Let's start by acknowledging that nothing (other than Krystal) is a White Castle substitute. When it comes to stinky steamy onion slider goodness, White Castle is tops. This is a love it or hate it affair. I am firmly in the love camp. If you don't love White Castle, you can get the hell out of my life.
I feel White Manna is the worst of the three. Yes, they cook their meat over onions, and it has that familiar slider form factor. The ambiance is dive perfection. However, their slider is the most burger-y of the choices here. It frankly just kind of tastes like a mediocre burger. This is the only place of the three that uses fresh onions in their burgers and there's almost no stinky onion steam flavor to speak of. It's more of a burger with grilled onions than a slider.
Finally, White Rose. Their burger packs the good old onion steam, but there are a couple of problems with the burger here. It's a little bland, and the meat to bun ratio is way off. However, all of these problems can be rectified by going double patty and adding bacon. In this final form (I prefer to go cali here), their burger is pretty awesome in my book. But I've brought many people here who just didn't 'get' it. If you want to be amazed, go here and order a side of bacon, regardless of what you get.