I have a Tern Verge X11 and a Helix Ultralight. I also tried the T-Line.
T-Line: I did not enjoy my ride. The shifting gears seemed cheap and I don't like the proprietary parts. However, it's the most compact folder. A must if you have to take the bus (but why would you if you have a bike?).
Verge X11: The best ride, the best performance. It folds quickly, but in a V shape (I found it too wide sometimes on a packed train). Moreover, it does not roll folded. Sure, there's a magnet but it's not strong enough and the wheels come quickly apart. There are a lot of accessories (Klickfix). The wheels are very thin (28-451) so not a good thing when it rains. I installed the mudguards for these wheels. I think I can use wider wheels but I would have to remove the mudguards. I want to keep the original wheels as they are part of the performance.
Helix Ultralight: A great compromise but with some disadvantages. It rides great, its fold is amazing (very compact), it rolls folded, it's light. The choice of wheels is indeed limited. It would have been great if you could use 1.75" tires. The other problem is the lack of accessories. If they manage to release mudguards and other accessories this year, this will be a big step forward for this small company. I use removable (rollable) mudguards, they do the job but Helix mudguards will be far better. It's a great bike.