I agree with Sophie (after all, she is a Pro). It doesn't matter what type of tile you use on your shower walls. All that matters is that your shower walls were made properly.

And today that means making sure that you get it as waterproof as possible. Never, ever use drywall, even so called water resistant drywall. It isn't meant for direct water contact. Use it on the rest of the bathroom.

Backer board like Hardie panels or similar are fine to use. Even better is foam backer board like Kerdi or Wedi or the new one by Durock. Just follow the steps required to ensure that you have a water tight install.

Standard ceramic will absorb 100% moisture.
Durabody ceramics will absorb 5% moisture.
Porcelain will absorb between 0.06-0.5% moisture.

Of course porcelain is the best suited product but your options are still open.
If you choose a ceramic for the walls, use an upgraded grout that does not stain. (this will stop moisture from absorbing into the sides and back of the tile (the areas of the tile that are not fire glazed.)
For the wet floor, I recommend using a standard grout so the floor can breath properly.

Thanks all. I will widen my search and include ceramic tile.

So we were initially going to install Kerdi (can't that go over drywall?) but our contractor, while willing to do it if we insist, is most comfortable with a different product (Densshield, a fiberglass backer) and feels Kerdi is unnecessary.

Should I insist on the Kerdi? Or go with what he's had most experience working with?

Cinar - was going to go with epoxy grout in the bathroom... wouldn't this help with water absorption issues more? How does Tec Power grout compare?

Just check the specs on the tile you like. I suspect more are good for wet areas than aren't.

https://www.ceramictilefoundation.org/homeowners-guide-to-hiring-qualified-tile-installer

If that detail is not done correctly, there goes the whole assembly. Verify how he plans to do that. Before the pan is done.

Kerdi is my preference. Here's some discussion on a contractor's board. Kerdi pan/floor also.

ALL tiles are ceramic. Porcelain is also "ceramic," just less absorbent but are just fine for your project. Yes, Kerdi can be used over standard drywall and is approved by Schluter. Done properly, water NEVER touches the drywall and I've done a buttload of them with never a callback...and, yes...I DO follow up.

@Avanti - yes, I was using porcelain and ceramic as shorthand as one does, to avoid the hassle of referencing it as "ceramic-porcelain" and "non-porcelain ceramic," etc.

Thanks all.

Choosing tile is not hard.Some tile sellers make it more complex than necessary by passing out a lot of misinformation.

Tile for a shower wall does not have to be particularly hard, but it must be reasonably waterproof. All commercial tile is tested for water absorption and the resulting grade is written somewhere on the box.

You should look for the ANSI Water Resistance rating. Water absorption is tested by weighing the tile, then boiling it for five hours. The tile is then soaked in cold water for another 24 hours. It is then weighed again to find its gain in weight from the original dry state. Any weight gain indicates water absorbed by the tile. The result is stated as a percent of change from its original dry weight in four classes:
Non-Vitreous
Water absorption of more than 7.0% by volume. Tile for non-wet areas. Around fire­places, for example. Typically intended for dry walls, hobby, and crafts use.
Semi-Vitreous
Water absorption of more than 3.0 percent, but not more than 7.0 percent.
Tile for areas that may get wet on occasion, but are unlikely to see constant or standing water. Back­spla­shes or countertops, for example.
Vitreous
Water absorption of more than 0.5 percent, but not more than 3.0 percent.
Virtually any indoor application including shower walls and floors. Outdoors in areas that do not freeze.
Imper­vious
Water absorption of 0.5 percent or less.
Any indoor or outdoor application.You are looking for tile rated Vitreous. But, keep in mind that Sophie Wheeler is absolutely right about the need for careful installation. Even properly rated tile will cause water problems down the road if it is poorly installed. We use the Kerdi installation method in preference to the older cementitious backer board method. It's a little faster and a lot more durable.

For more information on how to buy ceramic and porcelain tile, seePorcelaain vs. Ceramic Tile: Is there a difference?