I own a number of the Windrider II models, and one III model, and perhaps can share a little bit on them to assist. [edit: I was writing this when some of the above posts were made and before I saw them]
The Windrider II comes in four variants that I am aware of: standard (std), vented (vtd), extra vented (x vtd), and extra extra vented (xx vtd). The venting on the the vtd model is approximately 25% of the sail area, and, as you would expect, the percentage goes up with extra venting. Unlike the Revolution style of almost vertical panels of venting, the Freilein line has the vents in a different pattern as may be seen in the images on the Emma Kites website.
I got my kites from Emma Kites when it was still associated with AliExpress, however, I gather that Emma is no longer selling through that ebay-like venue, but, rather directly. When I bought my first kite there I had no experience with AliExpress, but they have an elaborate system of funds being held in escrow which appealed to me at the time as an additional level of comfort that the transaction will be completed properly. My transactions with Emma Kites have been entirely satisfactory although shipping does take quite a period of time in transit, probably not helped by these Covid times. I am in Canada, for what that is worth, and Emma is in China.
Another point to consider is whether to go with the PC20 cloth or PC31 cloth. I had the choice of either cloth in any of the II models and went with a mix in different kites. The former is supposedly lighter, although it does not seem to make a hugely material difference in overall weight (I have not checked this point). The PC31 is more like what we know as icarex (and may even be that, although they don’t say). As such it is less crinkly than PC20 in sound, and is perhaps less likely to hold creases. The price difference between the two sails was approximately US$40, but the kites are otherwise identical in terms of construction and materials used including fittings, bungees, spars, etc.
The spars used are called Crystal, and they are graduated in weight and strength, with the heaviest, being the T3 spar, being found in the highly vented kites. T2s are used in the standard version.
I also got the UL (not SUL) version of the next generation of Windrider (Windrider III). It sports an even lighter spar, the T1 as stated in the my review of that kite. The III versions also have different graphics, and some other changes as detailed in the aforesaid review. Mine is in PC31. This kite I actually got from a different vendor, also based in China, but on ebay.
I have a number of both Revolution and Freilein kites, quad and dual, and I would say that the construction quality of the Freileins that I have is probably a step above that of my Revolutions in terms of uniformity and detail.
In terms of durability, I have had most of these kites for over a year and they have been flown fairly extensively, up to about 10 hours each, and, so far, the durability has been excellent. I have neither busted a spar, had a stitch come undone, nor had any bits fall off!
I hope that this helps.