Karlovy Vary is two towns stitched together - the spa valley along the Tepla and the ordinary working town downstream - and where you sleep decides which one you wake up in.
The spa valley - Mlynske nabrezi and Stara Louka
The colonnades, the grand hotels and the springs, all within a few hundred metres. You are paying for the address and for silence after nine, because this end of town does not have a late night.
Around Vridlo and Divadelni namesti
The middle ground: still on the river and a couple of minutes from the Hot Spring Colonnade, but with normal shops and normal restaurants rather than spa boutiques. The best compromise if you want to walk everywhere.
T.G. Masaryka and the modern town
Downstream past the confluence, where the buses, the supermarkets and the ordinary prices are. Twenty minutes on foot or a short bus ride from the colonnades, and half the price of the valley.
Know which arrival you are booking for
Most coaches from Prague set you down at the terminal in the modern town, not in the spa valley, and the walk up the river with luggage is longer than the map suggests. If you are arriving late, book downstream and move up in the morning.