Ilianto, whatever you will buy, it will be a compromise. This is always the case with folding bike.

You says that you plan "to take big trips with my folder after some training, as well as everyday commuting".

If big trips means let's say 20-30 miles or more per day and include steep hills, the Dahon is probably a better choice of the two (larger wheels and wider gear range). On the other hand, if everyday commuting includes folding and taking on/off a buss or train than the Brompton is a better choice (more compact and rollable fold).

In general the Brompton has better quality. It has many proprietary parts but they are available as spare parts (e.g. see the Brompton section at http://www.sjscycles.co.uk/ ). The Dahon is in general of lower quality and some of the propriety parts are hard to find but is a decent bike and provides in general a good cost/features ration ('value').

(the opinion above are based on my personal experience with a 6sp Brompton S and Dahon Mu P24).

Another solution is to have two bikes, one that is optimized for long rides and one for short distance and commute. Some of us do exactly that.

Hope it helps.

Kam