Which is the best low-cost high-quality EEG device to write a sleep tracking validation paper about it?

We suggest to use Pi EEG 8 channel: https://www.crowdsupply.com/hackerbci/pieeg#products It costs less than Muse S in Switzerland has 8 instead of the 4 EEG channels of the Muse S. And we suppose that prices for Pi EEG remain stable. But as usual with EEG devices those which are interesting for costumers are not available right …