Looking at the Marlins' pitching roster, I see that Sanchez has the better ERA, while Nolasco has the better WHIP (if I calculated it correctly). Which would this group say is the better indicator to go by?

era would be a better indication, but even that does not always give a true indicator of future success...

Both are fine stats, far better than say, wins and losses. Neither is used much by sabremetricans because both are unduly influenced by factors beyond a pitchers control, such as luck and quality of the defense behind them. ERA contains home run rate in it, which is good thing. On the other hand, it has more statistical noise in it, especially in a small sample. If you are looking for better stats, try xFIP or Sierra.

I may be completely wrong but WHIP seems to involve less luck than ERA. Pitchers who limit baserunners tend to have more success than those who do not.