They're both execrable and never should have seen the light of day, Forest because of its reprehensible "hey, kids, it's a great idea to stop taking your meds" message, and Zygon Inversion because of its rather blatant xenophobia (especially with the Zygons being a thinly veiled Muslim metaphor).
I like The Twin Dilemma . I'd also watch ITFOTN 3-4 times in a row then the Zygon Two parter once, even though I hate ITFOTN a lot. A somewhat boring story, probably the second worse 6th Doctor story after Remembrance of the Daleks, but still watchable because of him. Then again, he's my favorite Doctor, so I'm obviously a bit biased toward his era.
The implication in that two-parter is that the minds are uploaded to Missy's Matrix knock-off at death (no idea how), so they can then be put back into the nano-built Cybermen.
That seems a bit convoluted. Yes, I know, a hallmark of Who villians, and villians in general. But, if they have the technology for nano-built Cybermen, why not just create them ready made with the proper programming rather than using the pattern from a person? Clearly they're not using the brain patterns because they want the Cybermen to have their own unique personalities!