I have both. But there is no crappy winter weather especially snow here so no comment on that.

The Swift and Brompton are far apart. Maybe as far apart as they can be.

The Brompton folds small, the Swift not. The folded Swift does have a short narrow footprint so fares well against a wall or bulkhead.
The Swift is much lighter but that does depend very much on your choice of componentry.
The Swift is much faster than the Brompton. At least 10%. Again that will be determined in part by your choice of componentry.
The ally Swift will withstand wet weather better than the steel Brompton. (The Ti one even better.)
If you want the Swift to be all season, it will like any bike need mudguards and suitable tyres.
The Swift geometry is same as a conventional medium bike. But riding no-hands is not feasible.
The Swift is stiffer than the Brompton.
The Swift will accommodate the fattest tyres available. The Brompton maximum is about 32mm.
The Swift frame is a like blank canvas. What you make from it is entirely up to you. The Brompton will forever be standard (within reason).