In the Datel application any
sealed automotive relay will be more than sufficient. In the case of a toggle switch, a metal framed,
sealed switch will typically have a longer actuation life over a totally plastic switch. The metal frame not only provides better pivot life, it also provides better support for the contact's snap action. Unfortunately the average consumer can't get information about the electrical characteristics of most over the counter electrical parts.
If you are looking for quality electrical switching and distribution gear and won't faint at the price difference between a $2 and a $6 switch, it is hard to beat marine grade parts that can be found at places like
this. The products are really weather proof and generally offer extended life as well as having good quality.
All things being equal, a higher amperage switch will have lower contact resistance (good) and a stronger snap action to hold the contacts solidly in, uhh, well, contact
When you select a switch for a more demanding job than a Datel meter be sure that the contact ratings have some 'head room' over the expected load. Selecting a 5 amp switch for a steady 5 amp job is asking for long term trouble, a 10 amp switch would be a better choice.
Edited to add: On a motorcycle's electrical system the difference of just TWO volts is major -- 14.0 volts = good; 12.0 volts = your day/trip is about to be ruined. You don't want to squander 0.1 to 0.4 volts just in your meter's hook-up. Fortunately the Datel's tiny current draw goes a very long way to preventing excess voltage drops.