Well considering the dell is the smallest 13" laptop on the market atm there's no arguing what wins that area.
It's worth noting in terms of size the 13 inch dell is about same as the 12 inch macbook because of the bezel-less display. If you want portability, again, I'd go with the dell. I've carried mine in my bag for almost a year and it's build like a brick but insanely lightweight actually.
'Downgrading' from a high-performance class macbook pro to a macbook 12" will no doubt be very noticable. 2012 wasn't too long ago and the processor in your 'old' laptop will be substantially faster. Storage however will be very fast and I can't argue with that at all.. Without going into too much detail, the processor will be multiple times slower.
In terms of ports, can I point out the dell has a USB C port, mini display port, as well as USB 3.0, while having the same footprint.
The macbook also has a terrible keyboard (it has almost zero travel) and so it's most likely to hurt your fingers while typing the essays etc that you mentioned earlier. "And if you didn't get on with its super-shallow keyboard, your fingers will remain as unconvinced as they were before - especially during long typing sessions." - techradar
Is avoiding the easy transition to windows really worth the
real drawbacks you'll be getting from staying with apple, especially when you're looking for portability
and speed that only one offers? I've had the same debate with 3 people now; they've bought it and couldn't be happier.
Also, carbon fibre is wayyyy more attractive than pink!
Let me know if there's anything else you're unsure about.