best is hard to define under the different metrics that both machines can be judged by,
the 2 mentioned machines are quite different beasts, one being a full domestic machine (breville) and the other being a semi pro build (Silvia).
i will try to point out a few differing points for you to consider as i have a little bit of exposure to those 2 models.
both are quite different machines.
The Brevile is the obvious winner here with built in fancy controls like PID control and pre-infusion, the brevile also has a built in grinder meaning you would need have the extra expense of a stand alone grinder.
secondly comes build quality, - for my money the silvia would be my preference with a brass 330ml boiler as opposed tho the ally thermoblock brevile machine. and a full brass 58mm portafilter as opposed to the smaller double walled consumer 53mm portafilter. the silvia weighs in at about 15kg where the breville is closer to 12kg.
there are plenty of v1 silvias that are operating fine 15 years on however if you dont adhere to a couple of basic rules you can boil them dry.
the silvia likely meets that description better than the BES870. but keep in mind we are comparing a $800 machine (silvia) with a $650 machine (BES870). some beleive these days that the silvia is a little overpriced when compared to other semi pro machines like the Lelit which are effectively based on the silvias design but now have features like PID control.I am very keen on a manual, no frills type of espresso machine.