Just my two cents here, YMMV. I have a high end Dahon (Speed Pro TT) as well as a Brompton M3L, and a bunch of other folders, so here goes.

They are entirely different in presentation and execution. Dahon has a 3x hub with widerange 9-sp cluster, so super wide gearing, while the Brommie has the BSR standard 3-speed hub with fairly close ratios, which I find quite limiting and especially lacking on the low end.

Dahon fold isn't anywhere near as quick, tidy or well-engineered as the Brompton. It seems almost ungainly in comparison.

Brompton uses more proprietary parts, Dahon uses more standard, although I hate the Dahon's KORE seatpost which uses its own saddle clamp - can't use standard saddles, have to use KORE saddles unless I swap the seatpost with a standard one.

Riding position is not optimal for me on the Brompton; I feel a bit stretched out. Dahon a bit more adjustable but not optimal.

I get the feeling that my Brommie will last much longer than the Dahon. This is subjective.

I like them both, but despite all the Brommie quirks, I still feel it's the better of two bikes, and definitely a tour-de-force of engineering.