a grinder is a grinder, if it gets the job done then its alright.
Space Case grinders have more "features" like a pollen catcher and i believe a storage container but if you just want a simple grinder go for a metal one.. ive broken a fair share of plastic grinders with tricky buds..
Space Case grinders are expensive, but after trying the cheaper grinders on ebay and my local headshop, I've found their worth the money. They have the smoothest grinding action, some grinders put their screen directly under the grinding chamber, I've found this can be troublesome, the screens are also typically not of the same quality. Look around Ebay for a real space case, I picked one up around christmas for 70, which also incuded the space case pollen press, which I also recommend.
I was looking at Space Case and Mendo Mulcher but decided on a Santa Cruz Shredder. I'll be receiving a 2-piece medium SC soon so I'll be able to compare the two grinders. I also have a Magic Flight Finishing Grinder that I've yet to use.