Where Samarkand's monuments are spread across a modern city, Bukhara's historic core survives largely intact and walkable — the Po-i-Kalyan complex, the Ark fortress, and a scatter of trading domes and madrasahs within a few hundred metres of each other around the Lyab-i Hauz pool.
Bukhara was a major centre of Islamic learning for centuries, and that history is visible in the sheer density of madrasahs packed into a compact old town. It's an easier city to see on foot than Samarkand, which makes it a good second stop after a more spread-out first day.