You can ride with panniers on a Swift. I have my Swift setup to easily do it. I just bought 2 Bromptons and I can tell you if I only had 1 bike it wouldn't be a Brompton! That big Brompton bag up front is a great air dam to slow you down. I have the 22 liter cloth pannier and it slows me down MUCH more than two panniers behind me on the Swift. If you want the Swift to ride great see my bike here:
They said it couldn’t be done! I am redefining the Swift little by little
If you want to see with rack and bag go here (but this is before I changed tires):
This has my rear rack on there with the bab that has fold down panniers as well. I have size 13 feet and if my feet don't hit the bags, only Shaq's will! The rack is quite useful and I often have grocery bag panniers on it or other types of bags or panniers on it. The Swift as I've built it is FAR faster, smoother, useful and more comfortable (ride quality and ergo's) than the Brompton. The only place where the Brompton trumps the Swift is in folded size.
I love how narrow the Swift is when folded in that it is easy to go through gates and turnstyles. It is taller but faster and less fiddling. I have also carried a ton of groceries on my Swift and my large frame with absolutely no issues. The brakes on the Bromptons suck compared to most. What model were you looking at? FYI- here is a review I just wrote about my Brompton purchase of 2 days ago (my detailed review after purchase is post #18):
Should or shouldn't I. Thinking about a Brompton
I would definitely spend more time riding bikes unless you have already made up your mind. I have no skin in the game so to speak and I have both the Brompton and Swift (and a few others) so I am pretty objective and not worshipping at the altar of any manufacturer. The Swift is the one bike that can really do it all with very little tweaking. The Brompton is more of a "one-trick-pony" but if you need that 1 trick nobody does it better...yet.