1. TSLA is close to it's all time high. SCTY is well below it's all time high (Granted, I don't see it going back to $80+ anytime).
2. TSLA market cap (34.5B) is 70% that of Ford (49.3B) or GM (49.7B). I know Ford and GM are probably undervalued and there are many differences between them and Tesla, but this suggests to me that Tesla is overvalued.
3. Solar City market cap is 2.4 billion. Solar City has a market share in the US of 34% (approx). Solar City market cap compared to the rest of the industry makes sense.
4. The Solar Industry is here to stay. Solar City, as the US leader, is not going bankrupt (the Tesla board of directors agrees with me on this or they would not approve the deal). Investments would swing back to SCTY at some point pushing the stock to $40-$50 (or ~4-5 billion in market cap).
I do agree with another poster that SolarCity was more likely to be heading towards $0, rather than back to $75.