The “Pros”:
1) Lower energy consumption because water is only heated when it’s needed.
2) Never run out of hot water (within design limits).
The “Cons”:
Aside from intial cost..........
1) Tankless must be sized for needs.
With a tank type heater, hot water can be used at any rate until the tank temperature drops. You can have several showers drawing water at the same time.
With a tankless, it will only heat water as per design. If your tankless is designed for 1 shower, it can not heat water fast enough to support more than that at the same time.
2) With GAS, consider the ignition system.
Standing pilot is always lit, but it is the cheapest and is foolproof.
Electronic ignition lights only on demand, but needs electricity to operate.
Hybrid ignition (usually a small built in water turbine) lights only on demand without electricity, but is most expensive and can be subject to hi-tech failures.
3) With ELECTRIC, requires much heavier electric supply than conventional water heater. Confirm your service panel can support load and cost of installing cables.