jaynos is saying that TSLA is unlikely to go much above $235, but Solar City had a lot of potential to move above $26.70. By converting them now, they've basically thrown away all the potential future movement.
The point is, that share prices are absolutely meaningless without the number of shares which just gives you market capitalization. Tesla could just do a 8:1 split and their per-share price would be $30/share -- Would that change their upside? Of course not.
Presumably jaynos meant those prices are relevant in relation to historical prices: Tesla is near all-time highs, while SolarCity's share price has been around $75 at one point.
I do agree with another poster that SolarCity was more likely to be heading towards $0, rather than back to $75.