What are you going to make with the spoke shave? EG if your doing chair legs or stick back spindles or chamfer's on straight board's, one with a flat sole would be ok, but if you want to do concave curves you will need one with a curved sole. Personally I prefer traditional wooden ones where the blade is held by friction in 2 small square mortice's; I like there simpilicity and the cutting blades are often quite wide up to 4 inches or so. But metal ones are usually easier to micro adjust for fine tolerance work, they sometimes have tiny screw adjusters.
A good dealer for quality 2nd hand tools (ie he sells absoulutely NO rubbish) is Tony Murland at Needham market (suffolk) I recently got a couple of new old stock wood spokeshaves for just £10, my other 2 were somewhat worn into a groove from doing spindle's