Both are 3.7V
batteries, but 3.7 is the "nominal" voltage, more of an average. Both
batteries start out at 4.2V fully charged, then gradually drop down to around 3.2V before demanding recharge. Since the 18650 has much higher mAh than the 14500, it stays at the higher end of that range (i.e., hit harder) longer.
The obvious advantage of the 14500 over the 18650 is its size, enabling a smaller PV, such as shown in these two versions of the Precious (the shortest
mod for each battery):
[For more detail on the differences between the 14500 and 18650, see
this recent thread in the Tips and Tricks forum.]
To have a "3.7V" mod (based on either of these batteries) hit harder than 4.2V, you can use an LR atomizer, which brings the watts (i.e., power or intensity of vape) up to the equivalent of a regular atty on a 5V mod.