It is a tough call but my vote would be for the mountain bike. I would also consider a used hardtail mountain bike. As previously mentioned newer ones are undervalued particularly used.
My reasoning is your concern is reliability. The issue with geared bikes in the rain is that everything just gets all gummed up (not including salt in this). Brake cables stick, derailleurs get stuck, ect. On a mountain bike these things can be fixed with tugging and hitting. And if it stays stuck you just have a single speed for a while