makeinu

Dahon vs Downtube:
-Dahons are usually much lighter. On paper Downtubes weigh about the same, but the numbers are not accurate (at least of the bikes I've tried...haven't heard complaints about the weight of the Mini though).
-If you don't know what you want at all then it's better to get a Downtube because they are cheaper (so no big deal if you make a mistake).
-If you basically know what you want it's better to get a Dahon since they are much nicer out of the box.
-If you have a very specific preference in components then Downtube is better because it uses more standard parts (so your specific upgrades are more likely to fit).
-As a smaller company Downtube has better factory direct customer service.
-Dahon has a better warranty.
-Dahons are more ubiquitous, which means easier test rides and more choice in bike shops to buy from (so, for example, it's easier to get a discount on MSRP or better in store service).

Downtubes should be serviceable at any shop, but for most usual maintenance issues so should Dahons.

As far as where to buy, brandscycle.com is one of the cheapest places to get a Dahon. Although they offer free shipping on most bikes they're also located right at the Wantagh LIRR station which is a 45 minute trip on the LIRR from Penn Station. If I were you I'd print out the brandscycle website, take a trip over there, test ride, ask them to price match the website (they will do this), have them assemble and tune (to lock in the warranty), and ride it home (using your closest LBS for future parts/maintenance). Just make sure they have the models you're most interested in in stock before going (because the website listing isn't necessarily representative of what they have on the show floor...for better and for worse).

I believe you still have 5 year warranty, but you need the LBS to get lifetime warranty...not sure how Dahon enforces this.