Depends on whether you are remodeling or this is new construction. Also depends on whether it is a first floor bath or upstairs and whether you have typical framed construction or on a cement slab.

If on a cement slab, then these contractors may be right. Typically you would "sink" the shower area so that you could do all of the proper preparation and at completion the tile in the shower would match up in height with the remainder of the floor. If you are on typical framed construction, then the floor joists can be cut down to achieve this - as long as your local building codes do not prevent this.

We design most showers to be curb-less. The shower floor typically is depressed 3" lower than the surrounding bath floor to allow the shower to drain properly. This is easy to do with new construction and more difficult (but possible) with renovations. See attached image of a curb-less shower from a renovation project east of Seattle.

--Nils Finne, AIA