Yep. If you make music just at home, buy a desktop PC. Just for the the upgradability alone.
If you are on the go, I'd go for regular laptop with active cooling. (Meaning a fan and a heatpipe.)
Those really slim laptops / tablet-laptop hybrids tend to have really small fan, or are even passive cooled. (Meaning the heatpipe is not cooled by fan, but by being connected to casing of the device.) What can happen is that eventhough you have i7 in there, it get's so hot inside that the processor reduces it's working frequency (thus power) in order to cool itself down, so it doesn'd damage itself. That's called throttling. Computing realtime multitrack audio with bunch of virtual sound sources is a quite demanding task, so you don't want throttling to be present too much. I didn't have the honor to use those Surfaces, but I'd be really careful while thinking about them for this job.
So if I were you and there was no option to go for desktop PC, I'd buy laptop that is not built so sleek, but has proper cooling inside. Gaming laptops are kind of cool for this, actually, as they are designed for heavier load, but don't cost you money of a proffesional workstation. ...they tend to be plastic-ish and bulky, though. But then it comes down to your priorities.

EDIT: To end it on positive note though, if you already have the Suface Pro, just test if it doesn't overheat under your usual load. (load some finished project and play it) Then if the CPU power is enough for you, you're good to go as you are.