It depends on my tasks, if i'm at home I use only desktop, in rest cases I use laptop because it's practice and turn wifi from my phone so i'm fine everywhere with it, problem is that if you're low on battery it is a problem to find a place to charge
The issue now there is two extremes. Power house Desktops & Thin Light Ultrabooks. That is an awesome combination if you can afford it. Big Laptop is a compromise for the people who need an all in one.
I don't own a desktop anymore so don't use those. I do have two laptops and an iPad. I typically been using the laptops more right now. Mostly cause I haven't had my iPad charged for a bit. XD And its easier to type on the laptops.
I use my laptop the most and that's because I'm almost always at home and it's on my desk in the family room. I think if I were away from home more I'd probably end up using my phone a lot more than I do now. I barely use any data on my phone LOL
Desktop for sure, it's pretty much my central hub that I do everything from.
When I'm out and about, it's my phone. I do have some old laptops lying around but I've struggled to every find a use for them.