Choose MerrySails if you want the cheapest direct-booked Bosphorus sunset cruise (€30 Mon/Tue/Thu), a 4-tier dinner ladder (€30–€90), a boutique yacht charter from €280, or fastest WhatsApp response (3 min avg).
Choose Bosphorustour if you want a daytime €10–€20 sightseeing sailing as a budget option, prefer to book through Viator because you already have OTA credit, or value a longer brand-recognition history in the Istanbul hotel-concierge channel.
Both are TÜRSAB A Group licensed — that's the same legal protection. The differences are operational: pricing transparency, package depth, yacht-charter availability, and customer-service response time.