SCSI or SSD?

How is the speed of the newest SSD compare to the fastest SCSI?

I assume you mean Serial Attached SCSI, right? I don't have any SSD, but unless you're in a workgroup/server type environment I don't think SAS would really benefit you as much as a SSD would

Do you mean the SAS SCSI are the fastest SCSI?

I was referring to something like this
http://www.newegg.com/Product/Product.aspx?Item=N82E16822116064

Depending on where the data is being read from on the spindle, the SCSI drive might be faster than one SSD by itself. Where the SSD will shine is in access time and sustained transfer across the entire capacity of the "disk." If you already have an Ultra 160 or Ultra 320 SCSI card, the capacity of the SCSI drive and it's generally good performance make it a pretty good choice. If you need to buy the card, I'd say forget it. The cost of a good SCSI controller at retail is high. I lucked into my controller. I get better performance than most SATA hard drives (the two/three platter high capacity drives are faster and have similar latency however) but I also ended up spending $60 total on the whole setup including shipping.

If I had the choice between an SSD and a 15K SCSI drive, controller, cables, and terminator at retail prices, I'd go for one or two of the really high speed SSDs.

15K SAS drives are a good solution. They're cheaper than SSDs by far and still quite high performance. However, if you're only interested in overall speed then the SSDs are your choice.