There are a number of SME arms from an apparently vast time period. I liked the SME V I once heard on a Pink Triangle PT TOO turntable, but when I was told the price of it I had to sit down for a while. The RB300 is a very good arm for the price, but it'll all depend which deck it's going as well. I've only ever known one person try and fit one to a Linn LP12 for instance and that was a challange.
Is the SME going to be new? Are you lookin at a new Rega arm as opposed to a secondhand SME perhaps? Secondhand arms are something I've always shyed away from rightly or wrongly - don't know enough about them.
Cheers