I'm about to start a new commission. It's 14x19" so half a sheet, of three black labs. The client really likes the one of them all laid down together - it's actually a photo of them in the back of a car, and I've just added a dark grey background. This one is absolutely gorgeous as a photo - but there is little detail so it would all be about the tonal values and getting those lights and darks spot on - quite dramatic I think. I would create this one on anthracite pastelmat. The other two are photos she's had taken - the quality is OK, lots of green reflections in the fur which I wouldn't include. I think on the black/dark grey it looks really quite dramatic again, and I like the white one too.
I spend quite a bit of time in photoshop composing pieces like this and working out what sort of background if any would look best. I use photoshop as that's what I'm used to, and I have a monthly subscription which is about $9.
If she goes for the three heads, I will spend more time in lightroom re touching them as they are currently too shiny. I'd want to knock the bright highlights back a little and deepen the shadows.
When I send these images to the client, I make sure that it's clear what I'd leave out, and that they are in fact photos - I've had someone come back to me saying wow that was quick, looks amazing lol!! I also point out where I wouldn't be able to create details - like the top pieces - that way they can make a decision as to what they would like. Personally I'd really like to the do the one at the top - I've done a similar one recently of two black labs running and it worked really really well - plus it'll be much quicker to do this one ;)