They fold to similar size (yes, the brompton fold a wee bit smaller), have similar size wheel (305 erd vs 349 erd). Personally, I liked the folding mechanism better on the Dahon. Nothing against the brompton, I might get a custom one with 5 speed SA hub for the heck of it but I like the Dahon more when I tested both bikes out.
Honestly I was just wondering why you felt this way. In my eyes they have far more differences, each with their pros and cons, than commonalities that IMHO could "break the deal" either way. From what I see they differ in:
- choice of frame metal
- fold design
- the D3 has height adjustable handlebars where as an M, P or H type Brommie allows for fore/aft adjustment
- folding pedal design
- the Brommie's frame incorporates a suspension rear
- brake design (V vs double pivot caliper)
- as you pointed out, wheel size (a concern when needing replacement tubes or tires... the 349 being not as readily available at all bike shops if you should need them NOW)
I'm sure there are plenty more but I think the differences I listed are enough points to ponder for the cyclist in the market to purchase either of the two (or of either ilk). And the fact that you chose the D3 over a Brompton and I chose a Brompton over a mid-hinge aluminum folder attests to the weight of these differences.