What if the choices were between:
H2CN-
SH-
Would SH- still win out because Sulfur is a larger atom?
but 2 things are going for H2CN:
1.) larger molecule size
2.) N is much more electronegative
You mean CN-, right? I think what medpr said might be misleading, too. Because you don't just look at the number of atoms attached to a molecule as that may contribute to stability or take away from stability. For example, trifluoroacetic acid is more less due to the extra F's (becomes more acidic), but if you add methyl groups instead of F's then it becomes MORE stable (less acidic). Inductive effect depends on whether the introduced groups are electron withdrawing (F) or donating (alkyl). Withdrawing stabilize negative charges, making stuff more acidic. Donating stabilizes positive charges, making stuff less acidic.
That said, this question sucks because they are really close in acidity. H3CCH2SH is like 10, HCN is like 9. I would like to be corrected if i'm wrong, but I don't think there is any way to figure this out logically unless you know the pKa values.
So to answer your question, we're looking for the species that exists the most stably when it leaves. So the answer is CN-. Comparing this to the original question, CN- exists more stably in solution than H3CCH2S- because H3CCH2S- is more basic. H3CCH2SH exists more stably in solution than HCN because HCN is more acidic.
Also, for contrast, H2S pKa is like 6. So adding that ethyl group to the sulfur makes it much less acidic.