All I can tell you is that I have three Reliable SewQuiet servo motors on three different sewing machines, and I have not had the tiniest bit of trouble with any of them. I have two SewQuiet 3000s and one SewQuiet 4000.
The new SewQuiet 5000 is better than either of the others. The 5000 is available for $199 with shipping included. CLICK HERE
For me, I'd rather have a servo motor I know is going to work than the extra $80. I can't even find any info about the Family FES 550N.