For your Alto, it is best to stick with either the W16EXU or the BP5ES, whichever you can find easier and cheaper.
Any "improved" spark plugs will not cause any appreciable improvement in performance of this engine because it is not designed for that.
The only exception is if you have changes other things already: for example if you have leaned out the mixture, or if you do steady high speed running on the highway under load, the engine will run hotter, and you will need to move down the heat range (W20EXU or BP6ES).
Similarly, if you runs mostly around town at low speeds, or if you run a richer mixture for performance, then moving up the heat range (W14EXU or BP4ES) will give you longer plug life and less fouling.
Sometimes if there is pre-detonation due to carbon build up in a older engine, moving to a colder plug may help performance.
Please note that "hot" and "cold" plugs refer to the design characteristics of a plug that determine the temperature of the tip of the spark plug depending on how quickly heat can be transferred away.
Thus, you will see that a hotter plug has a thinner ceramic insulator so that less heat is conducted away and the tip remains hotter, whereas a colder plug will have a thicker ceramic insulator that conducts more heat away from the tip so that it remains cooler.
Keep in mind that the heat range of a plug has only a minimal effect on the combustion temperature itself.
Quite a lot of "high performance" and multi-tip plugs do not result in any improvements in a car engine like the one in the Alto.