Depends (again ;-)) Wood-Engineered for floorboards, 10mm solid for parquet floor (my absolute favourite).
whats your reasons behind that? For me its engineered also, reason being its more stable and looks the best in my eyes with the season changes.
Reasons for W-E, mainly indeed for stability - especially important here in the UK where house climate control though in house plants and ventilation is still "foreign" to most. Plus it takes less hardwood trees and since more and more manufacturers are switching from solid to W-E (less trees and less complaints) the variety in choices gets larger and larger. (Plus of course there's a quality W-E for every budget, from simple 3-strip Oak Rustic to very expensive 20mm multilayered Wenge) Why design parquet? Because every single floor - more than a floorboard floor - is unique, and installing it is still a craft.
