I purchased a compressor from Vintage Air about 10 years ago and had the
same discussion with them.  They said the SD709 was more efficient and was a
"superior compressor".  I bought the SD709 to replace a SD508 and found the
SD709 was also much quieter and had much less tendency to stall when
starting because it had more pistons that were smaller in displacement.  The
SD709 is just a better compressor and if the SD508 was in unknown condition
I would go for the SD709.

The www.sanden.com web site has some useful information and performance graphs.