Depends how much gear range you need, just flat land and minor hills, or steep hills and want to be able to spin up if they are long. Without an IGH (or Schlumpf drive at the crank), it is extremely difficult or impossible to get a wide gearing range with just derailleurs on 349 wheels, and even with IGH, Brompton uses a special "wide range" IGH. In the past, you didn't have a wide enough range on the cogs. A 58T chainring is not much larger than 52 in percent. Some have mounted 1X cassettes with like 11-42 on 349s using newer advanced rear derailleurs so they don't drag on the ground. I also met a fellow with a Brompton with a Schlumpf drive. Go to one of the online gear calculators and run calculations, like sheldon brown gear calc. For me, I have a 2X derailleur system on my 20" wheel folder with 400% range, 21-85 gear inches and I can handle steep hills and 85 is all I need for a high, I'm not racing. If I heavy tour I may go up to 42 cog to get 566% range and a 15 inch low. Run numbers for both bikes on gear calc and compare to gearing on known bikes you like. Also depends on your riding distances, like you said, the Brompton does not ride great. Consider if any flying travel and baggage implications. Consider if you want pannier capability.