The changes to the Spheros FB are:
New/Improved spool design that supposedly allows the line to be wound more efficiently, resulting in better casting.
New/Updated worm/pinion gear that works with the redesigned spool to lay line better.
There are a couple of other "improvements", I just don't remember what they are.
The Spheros you see at a discounted price are the older Spheros FA's. Just as a reminder, Shimano drops a reel from its service program 5 years after it's dropped from production. So those folks with Spheros FA's -- that less than 4 years from now.
If I were fishing with braid, I'd get the FB. If fishing with mono, the discounted FA would probably get purchased and write it off once it started to fail.
PB |