Weatherproofed Connector For Outdoor Valve
Heat-shrink tubing does not always hermetically seal a connection. The advantage of the wire nuts is that they are filled with silicone (not silicon) grease that prevents water from corroding the connection.

What I have done is to make my solder connection and then cover it liberally with a high-quality silicone or urethane caulk. Put shrink tubing over that. When you shrink the tubing it forces the caulk into all of the little gaps and gives a better seal than shrink tubing alone.