For this big an investment, it does pay to shop carefully. I didn't do any comparison when I bought my first good new bike, I should have.

If road surfaces are rough and unavoidable, you might want a folder with full suspension.

OK, I understand about tight space. I have a folder, but seldom fold it as it requires removing the panniers. But space is tight, I'm sure tighter than you. I have a 5'x18" table against the wall, I store the bike upright on it to keep it out of the walkway, and it is also my repair table with the bike inverted. Since you would be what I categorize as a "frequent folder", you need a bike conducive to that. My bifold Dahon doesn't really take longer to fold than a Brompton or Helix, it's just a lot more bulky when folded and difficult to carry. I wheel it fully open unfolded through stores, they are understanding of that due to rampant bike theft here now. At grocery store I just carry their hand basket, but at goodwill, they know me, I just put the goods in the panniers and pull out at checkout, then repack. I have front and rear panniers so I can really lug a lot of groceries, plus room to strap down a 6-pack on top of the front rack, and even hang bags over the handlebars if needed. So based on the above:

- Check your closet space versus Brompton and Helix, especially the orientation you want it to fit. You should be able to find exact folded sizes online. But at the same time, if you don't need to carry the folded bike far, you may also want to look at a bifold 20" wheel folder, available for $500 new for decent quality, to begin with. But if you need to carry or wheel the bike folded some distance, sounds like Brompton would win, it does both well. I don't think the Helix wheels well with the tires up against the frame tubes, not certain, and I think it might be heavier, check the specs. And a bifold like my old Dahon Speed 7 doesn't wheel well folded, though I've done it short distances on the ferry, but it requires removing the panniers; I instead pay $1 more and put it unfolded on the car deck.

- Decide your cargo requirements; The Brompton has a low rear rack, I have seen some put panniers on it, but not full-size ones without a tall rack, which destroys the folding design. Most people touring with a Brompton have the extra-large handlebar bag, plus a medium sized (2000-3000 cubic inch) backpack sitting upright on the rear rack and strapped to the seatpost. Helix, there are videos of people touring with them with a folding rear rack, but that needs to be attached and detached with screws, OK for long travel but unusable to do daily, or leave attached without destroying the folding ability, just like a Brompton (both have "swinging" rear triangles). That is one of the few upsides of my Dahon Speed, because it bifolds, the rack stays on top and I can fold the bike with it in place, even with my tall (large wheel) rack to hold full-size panniers. Dahon no longer makes the Speed model, the Mariner is same size folded but is a *terrible* deal now, the price doubled, but there are other comparable bifold 20" wheel bikes at half that price now. But for folded size comparison, you can use Dahon Mariner dimensions. I think I'll include some pics of my folder so you have an idea.

In use (notice the additional shopping bag hung from the aero bars, and rear rack and panniers well aft for heel clearance, and double crank which I added (not standard)), and folded on the train; I can easily do 20 miles on the bike and longer, key is that I have the size issues really dialed in, multiple hand positions plus aero bars, etc. Ride quality is not ideal, I need to put larger 2" section tires on it to improve that.