There are so many variables and idiosyncracies in folding bikes that I consider it imperative to try a bunch before you settle on one. In particular:
1. Speed of fold.
2. Ease of fold.
3. Quality of folding bits. Is there play in hinges, etc.?
4. Weight of bike.
5. Can you roll the bike in the folded or semi-folded position?
6. If you can, how easy is it to roll? Some are easy, some are not.
7. Quality of components.
8. Quality of frame.
9. How compact is the bike when completely folded?
10. If you need to carry the bike any distance (and this becomes an issue if you're in an area where a bike can't be rolled along unfolded - trust me, these situations do exist!), is it comfortable, and how far can you go?
11. How many parts are proprietary? The fewer the better, as you might be out of service if it's a critical part that needs replacing only by the manufacturer, for ex., the main frame hinge.
12. What kind of warranty on frame & folding bits? Some manufacturers offer a lifetime warranty against defects, others quite limited.
13. Is the fit comfortable for you, and if it's not, can it be modified (different stem mast, stems, etc.) to be a reasonable fit for you? Because if the bike isn't comfortable for you to ride, you will ride it only when you have to, as you instinctively know the longer you ride it, the more uncomfortable you'll get.
14. Is there a way to secure the entire folded bike together - straps, magnets and the like - or does it just flop around when you carry it?