Actually, the 304 has a compliance of 14, definitely medium compliance, and should be a great match for the RB300. I own a 103R and can vouch that it needs a high mass arm to sound its best. You can mod the RB300 to work pretty well with the 103R and it will sound terrific. The 304 is twice the price at retail, but that doesn't necessarily mean it's better. It'll be interesting to hear from people who've heard both.
David
David