For acid stained concrete, the best answer would be to use both!
The typical recommendation is to first apply a clear acrylic sealer, either solvent-based or water-based. (The contractor needs to make sure that the acid stained concrete is properly and completely neutralized PRIOR to the application of the sealer.) The sealer can be spray applied or applied with a lint-free roller.
Once the sealer is completely dry, an application of a floor finish or wax is recommended to protect the sealer from wear. (Allow adequate time for a water-based sealer to completely dry to prevent blushing.)
Floor finishes are typically water-based acrylic compositions that can be applied over the previously applied sealer. The waxes can be various compositions (carnuba wax, beeswax, blends of the two, sometimes in solvents) that are usually applied with a heavy-duty floor-buffing machine. All of these products have exceptional resistance to scratches, scuffing and black heel marks.
Most installers protect a stained floor with a minimum of six coats (sometimes up to 10 coats) of a floor finish or wax. The theory is the floor finish or wax is sacrificial, and can be re-applied as needed. The floor owner must be diligent about continued maintenance and re-application of the floor finish or wax. The floor finish or wax should never be allowed to wear down to the sealer, as the sealer will not be resistant to scratches, buffing, etc.
Both floor finishes and waxes can be purchased at janitorial supply houses or from chemical stain manufacturers.
NOTE: Due to the number of floor finishes and waxes available, a test patch is ALWAYS recommended prior to a full application.