Can't help directly with your decision but maybe I can provide some info that may help.
I have a pair of Dali Mentor 2s and wondered if the Helicon MK1 was an upgrade from the Mentor 2. Here is my question sent to Dali and a response form Sound Org, US distributor for Dal.
Q: What are the differences (improvements) from the Helicon 300 MK1 to Helicon 300 MK2. Also, what are the differences (improvements) of the Helicon 300 MK 1 vs the Mentor 2?
A: Thank you for your interest in DALI. Your inquiry was forwarded on to us since we are the US importer for DALI. I'm happy to answer your questions.
The MK2 versions on the Helicon range of speakers had a number of technical and cosmetic improvements. The first is, that the drivers for the range of speakers were redesigned and improved for greater control and clarity. This also was accompanied by changes in the crossovers to take advantage of the new drivers. This yielded an overall improvement in sound that has been praised by critics and consumers alike. They are no more open with greater extension both high and low. They cosmetic changes include the new high gloss finish with black baffles that resemble the appearance of the Euphonia series speakers.
The Mentor series speakers occupy the performance class just below the Helicon series. They are newer designs than the older MK1 Helicon and in some ways do have technical advantages. The Mentor 2 would very closely approach the original Helicon 300 in overall performance and even exceed it in a few areas. The advantage is you could likely buy a new pair of Mentor 2s for about the same price of used Helicon 300 MK1 and have these advantages and a warranty. Go to your local DALI retailer to listen to some speakers and decide for yourself.