"ECC and non-ECC un-buffered DIMM."
I assume you mean "ECC
or Non-ECC"? The STRIX X870-I GAMING WIFI can support both ECC RAM modules
and non-ECC RAM modules, but there's no such thing as "an ECC and non-ECC un-buffered DIMM" because it's a contradiction (a DIMM is either ECC or not. It can't somehow be both ECC and Non-ECC at the same time), and you can't mix ECC and Non-ECC DIMMs in the same PC.
The Kingston RAM is ECC. The Nemix RAM is Non-ECC (hence why it's much cheaper). If stability is your top priority, the Nemix RAM would not be suitable.
I would be wary of using ECC RAM in an ASUS X870 motherboard.
While ECC RAM is compatible with ASUS X870/X870E motherboards, some users have found that the ECC functionality is disabled on some models. It may be possible to enable full ECC support in the motherboard's BIOS settings, or by updating the BIOS version, but if you cannot get the ECC error correction to work, there would be no reason to use ECC RAM, as it would be a waste of money compared to normal non-ECC DDR5 RAM.
(I assume it's ok to post this link for evidentiary/explanatory purposes. If not, please let me know, mods)
If you want ECC RAM, I would personally opt for a different AM5 motherboard which is confirmed to have working ECC functionality. There are a few different models which people have reported having working error correction on reddit and other sites, but I couldn't find any talking about the STRIX X870-I GAMING WIFI. If anyone with an STRIX X870-I GAMING WIFI motherboard has been able to get full/multi-bit/side-band RAM error correction working, it would be very useful to get confirmation. It also might be possible to ask an ASUS rep to confirm whether full ECC error correction can be enabled on this board, and whether you might need a specific BIOS version, though some reps don't understand the difference between the different levels of ECC support, so might give you a misleading answer. On-die ECC is included on all DDR5 modules (even "non-ECC" DDR5 has on-die ECC) and is supported on all DDR5 motherboards, but full ECC support is only available on specific motherboards with specific BIOS versions. A motherboard having ECC RAM module compatibility listed on the manufacturer's website means that it is able to run with ECC modules installed, but doesn't necessarily mean that full ECC is supported.