I have both so things you should know:
A rostfrei is a heavy thick handled bali. It has a pretty sharp blade but does not hold a good edge.
Particularly The Wave rostfrei, the one with solid handles with wave design(comes in black and chrome), is the one I have. The wave design is supposed to make the handles easier to spin in your hands. It does just that, but I think a little too much. The handles spin when you don't want them too. And I also think that it's a little bit too heavy for me. The blade has a mirror finish. It wobbles but not as much as other rostfrei's and takes about 2 weeks before you really notice the wobble IMO.
My freind has the silver dotted black handled rostfrei. It is slightly lighter and has a duller blade with a satin finish. This bali gets really wobbly after a week or so.
The rostfrei balis are probably the best balisongs you can get for under the price of a Jaguar(around $20) as you can get a rosfrei for around $10. I think they are good for the price since where I get mine the china made cheapies are the same price and are really POS. Stay away from those.
The only thing you have to remember is that the BM42, or any other high end bali that you will probably end up getting and never put down, has the thin handled design and will be lighter than the rostfrei's.
But I guess it has its advantage since you can think of the rostfrei's big heavy handles as training for your hands and arms. When you get the 42 you'll adjust to the weight and size but retain the strength to manipulate fast and furious
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"May your blade chip and shatter..." - Paul Atreides,
Frank Herbert's Dune