Borewell or farm pond, which water source should I invest in for irrigation?

These two solve different problems so the right answer depends on your situation. A borewell taps groundwater and gives you water on demand at any time, which is convenient, but it depends on the water table, can fail if the area is over exploited, the depth and cost are uncertain before drilling, and yield often drops over years if recharge is poor. A farm pond stores rainwater and your own runoff, so it does not deplete groundwater, it actually helps recharge, it gives a buffer for life saving irrigation in dry spells, and it can store canal water that comes only on rotation; the limits are that it loses water to evaporation and seepage (a lining cuts seepage but adds cost), it occupies land, and it only fills as much as the rain and catchment allow. Many farmers do best with both: a pond to capture and store, and a smaller dependence on the bore, with the pond also recharging the bore. Both are supported under PMKSY: farm ponds under the watershed component and micro irrigation to use the stored water efficiently. Costs vary a lot with pond size, lining, and bore depth, so get local estimates. Confirm the current farm pond and water harvesting subsidy and the procedure on your state PMKSY or watershed portal before you dig.