b&w805s vs ATC scm19

Based on your preference ATC is the far better choice. However many people prefer the BBC dip in the B&W midrange and the bass extension. The B&W is a more typical impressive sounding design that audiophiles would tend to go for over the precise sound of ATC. If you want to hear truth and a midrange that is a near to the clarity/transparency of the best electrostatics then go ATC. This midrange is used by Telarc and many others. You will enjoy it but your friends may remark on the lack of boom and sizzle. If you suspect a tendency yourself towards plenty of bass and scintillating highs and want a really cool looking speaker then B&W 805S is much more in tune with the "spices" that audiophiles tend to like the taste of.

I´ll be in mind Shadome.Do you think both can be a good combo for A/B material and take decitions?

I never heard ATC speakers. However, as far as I know, there are studios who utilize B&W speakers for monitoring. Lucasfilm THX being one of them. Or at least they used to be.
In any case, it looks like you're looking for a nice monitors. if you have a powerful amplifier(capable, with quality power not just numbers, around 150w/ch and up) you may want to consider Totem Mani-2 Signature. These are full range monitors that go down into 20s in the bass. Real good sounding speaker. Revealing. uses Dynaudio drivers. Very impressive speaker. My second choice would be the 805S. You will hear plenty of detail with either speaker. B&Ws however may sound a touch fuller in the mids.
JMO.

Interesting those totem´s.
Is Totem 1 at the same level?
Maybe would be another option with sub.

Mani-2 sounds like a floorstander. Except it is a monitor. Accurate and musical at the same time.
I never heard Totem 1. If you have the proper amp, I'd go with Mani-2 Sig instead of anything else. You most likely won't need a sub. But remember, amplifier will be the key. If you have a lesser quality amp, then you may want to consider other speaker. In which case B&Ws are out of the picture as well. They too need a very good amplification. With B&Ws, if you don't have proper amplification, they may sound ugly.

Now I´m using bel canto s300.

should be OK as far as power is concerned.