After pricing the outlets, it's appliance modules, all the way, for me for on/off Z-wave devices... Dimmers are a different story. The pricing on those is about even. You're looking at $100+ US for outlets and $45+ for appliance modules, easy. With dimmers, the $45+ range for all of them is pretty much where you are. I can't cost justify (one Z-wave controlled plug in a) receptacle for more than double the price.
Plus, once the Z-wave receptacle is in,... you're done,... You can't move it. For a Light switch, it's different, since light switches are always for controlling lighting. It doesn't matter whether it is overhead or controlling an outlet. But a receptacle can go either way,... lighting or some other appliance or device. So, the thing with receptacles is that a room can always be subject to rearrangement, especially floor and tabletop lighting. Light switches are permanent fixtures. So in that sense I don't see the downside of appliance modules and plug in dimmers.
Now, obviously with Z-wave, you don't want to go moving devices around too much, as it affects the network. But, when you've got a wife like mine, that is constantly rearranging the furniture and painting/remodeling, not having flexibility in terms of the outlets is not an option. She even put a nail through her foot this weekend with all her remodeling.