It's a matter of whether you can live without the extra features that the S558i has. These features include a completely seemless electrical connection from the cannister to the power nozzle without "any" wires exposed. One has complete control of the motor speed from your fingertips at the end of the hose rather than adjusting at the cannister. One can also turn off the machine from the hose end without going directly to the cannister.  
 
The Miele S514i Solaris Electro Plus has the same power and air flow as the more expensive S558i. The super air clean filter does an excellent job at filtration and you could always add the HEPA filter in the future if you really need it. I think the S514i is the most vacuum for the money, around $700.00. The S558i is around $1000.00, when you add the SEB217-2 power nozzle. This same basic nozzle is included with the Solaris. I purchased the S658 Blue Moon which is the full sized version of the Silver Moon. The machine is outstanding, quiet and powerful with air flow ratings of 140 CFM (like the S514i & S558i). I love the way the hose rotates a full 360 degrees at "both" ends of the hose. One can not get tangled up with these fine machines.  
 
If you can live without the extras, the S514i is an excellent choice and will do the end result just as well as the top of the line Blue Moon. I bought mine from Ristenbatt Vacuum. Check them out at their web site ling below. They are great people to deal with and have extensive technical information on the Miele line in addition to other fine brands.
 
 
 
Related link: <a href="http://www.ristenbatt.com">Miele Dealer</a>