If it helps, it's been discussed here beofre, and the "in a nutshell" is that an archback brings out a lushness when strummed that's not possible with flatback. The shape emphasizes overtones and prolongs sustain, which may also be desirable when playing scales/solos.
A flatback is thought to be more desirable for fingerpicking where individual note definition is sought, "unclouded" by other notes still sustaining in the body.
It also has a focused, more "punchy" sound when playing chords.
Ask the guy who has one of each: me.
I'd sell the D40 before the D25, but then I'll never sell the D25.
Some of that's emotional, but the D25 is flat-out more versatile for me.
I do like the '40 more and more as time goes on, and after a few years of ownership, the analogy I came up with is that sonically an archback's like a parachute flare, lights up everything from above, but a flatbacks's like a searchlight, a tight focused beam.