I am modifying a computer for some heavy engineering data processing. The system is I7-9700.
I see that 970 PRO price came down to $200 and I don't see any reason not to choose 970 PRO at this point.
I assume MLC NAND should have better reliability in terms of data retention, along with out-of-cache performance - am I right? Any reason I should pick 980 PRO which is $20 cheaper right now?
Yes, MLC has better write endurance and data retention. Whether or not that matters is a different discussion - obviously you should get a drive with capacitors if you want power loss protection, for example, and the system should have UPS as well. Backups and redundancy are also good. With those in place, it's unlikely you would notice the difference between MLC and TLC these days...
MLC has a simpler programming scheme/sequence and larger room for error due to larger voltage thresholds. If you're constantly rewriting this isn't a big deal outside of data-in-flight protection. Without capacitors, data-in-flight is always at risk but moreso with TLC than MLC, technically. I say technically because TLC drives will write to SLC first before moving the data over which is even more secure than raw MLC. When moving from SLC to TLC, or if you're writing straight to TLC (if the SLC cache is exhausted), data-in-flight is at higher risk due to higher program times for upper page values and more sensitive voltage thresholds.
I could go on with the technical details but I have to be honest - the 970 PRO isn't particularly worthwhile outside of niche use, and I'm not sure you fit that niche. I would suggest dual drives in a mirror, OEM enterprise/DC with capacitors, regardless of NAND type. This can be pretty affordable (Timetec OEM in 20110 was like $70 for 960GB recently) but performance varies.