Warped bed - bulge vs dip?
That depends on the use. If you have a slate of glass a dip might be better (but the heat transfer at the dip would not be ideal), a bulge would stress and tilt the glass.

If you print directly onto the metal build surface it would not matter if you use a(n automatic) bed levelling system because you can compensate for the bulge or the dip using a mesh of the bed. Without a scan of the build platform you would have issues in getting the filament to stick.

Basically, there is not a better solution, best solution is to get a flat bed if the bulge or dip are making printing difficult.