There are some other differences -- the RB301, for example, uses a different antiskating adjuster than the old RB300, and the cable is improved.
I haven't tried the new Rega arms yet, but I have owned my share of RB250 and RB300 arms. I tend to prefer the RB250.
I've never been convinced the RB300 is any better, and in fact, some argue that the RB250 is actually the better arm regardless of price.
If you're using the arm for a second cartridge for playing junk records or everyday stuff, you can probably find an RB100 even cheaper.
Ekobesky (Reviews | Answers | This Thread)