Snap caps are chosen strictly for convenience in ease of applying the cap to the vial, such as in the case of capping hundreds of samples at a time.
Screw caps on the other hand provide a better seal (when the cap is properly matched to the vial threading) and is therefore more reliable.
An important thing to consider is the solvent or diluent in the vial; if volatile solvents are used, screw caps are recommended because of the better seal. If the solvent or diluent is water and not being heated above room temperature, the snap cap may be adequate. In the case of the sample, it does not matter which type of cap is chosen as the sample should never come in contact with the cap and should be unaffected except for the points discussed previously.
