As someone who owned (and maintained) a DA42-VI for 4 years, and now owns a DA62 for 3.5 years (and sells both), here's how they compare:
- The performance is nearly identical. My DA42-VI (pre-100 kg weight increase) maybe climbed about 100 fpm better than the DA62 because it was much lighter: 1900 kg vs. 2300 kg max gross weight. However newer DA42-VIs with a higher 1999 kg max gross weight perform the same as a DA62 when loaded. With either plane you'll see 1000 fpm climbs all the way into the teens. It's about a 5 minute checkout in a DA62 if you've flown a DA42-VI because they fly so similarly.
- The DA62 burns 1 gph more per side in cruise (15 gph vs. 13 gph total). But the DA62 has bigger auxiliary fuel tanks (18 gal vs. 13 gal each on the DA42), so the range is very similar.
- The DA62 has about 300 lbs more payload when equipped the same as a DA42-VI. Think of the DA42-VI as a 3-person airplane, whereas the DA62 will comfortably carry 4-5 people and their stuff. If you want to take another couple along in a DA62, you don't usually need to ask what they weigh.
- The DA62 is easier to work on, because the avionics and mechanical systems (essentially identical to the DA42-VI) are more readily accessible. Moving the G1000 avionics bay and gear hydraulic system to the DA62 nose is a big improvement vs. the DA42-VI with everything crammed into the back fuselage, especially with air conditioning. Replacing the battery is easier on the DA62 because you don't have to remove the TKS fluid tank to get to it.
- The DA42-VI has a strip of plastic "speed tape" on each side of the rudder hinge that repeatedly cracked and broke off in flight (not needed on the DA62). The DA42-VI (like all DA42s) has a stick limiter that has to be pre-flight checked; the DA62 doesn't have or need one.
- The DA62 interior is big enough for 300+ lb and/or 6' 8" pilots that won't comfortably or otherwise fit in a DA42.
- Fit and finish are the same, especially now that the DA42-VI also can be painted multiple colors. Both aircraft share the same wings (except tips) and engine nacelles, and both are mostly constructed of carbon fiber (CFRP) with the same build processes.
Bottom line: If you're big/tall/heavy and or need to be able to carry 2 couples, get a DA62. Otherwise you'll be very happy with a DA42-VI.