Even though the paper Hardness and Yield Strength of CO2 Ice Under Martian Temperature Conditions concentrates on carbon dioxide ice it also compares the results with water ice. The strength properties of carbon dioxide ice are slightly lower than those for water ice.
The paper confirms that ice hardness increases with the reduction of temperature.
Concerning the question of what would be the most cost effective way to remove and collect subsurface [water] ice on Mars? The answer would be, ... it depends! It would depend on the temperature (analogous to hardness) of the ice, the thickness of the ice and how much ice would be removed in each pass of removal. It would also depend on whether there was the desire and means to thermally soften the ice before harvesting and if so, to what temperature should the ice be heated, but still remain solid, prior to harvesting. It would also depend on how much wastage of ice would be tolerated and how gentle or aggressive the process needed to be.
Aggressive blasting could send fragments of ice flying everywhere leading to highly fragmented ice, but also ice that could be lost and never utilized. A gentler blasting process where less explosive is used, on a wider blasting pattern, would result in less fragmentation and less fly-ice. The degree of fly-ice could be reduced by the use of blasting mats or covering the ice with additional finer regolith prior to blasting. By finer regolith I mean material that is reasonably uniform in size, no large rocks or boulders and can be easily moved and removed.
The benefits of cutting or sawing are the minimization of losses due to fragmentation of the ice. Chipping/Chiseling would essentially produce fragments that need to be shoveled or collected by a gathering arm loader. It could also be very tedious, but it has the advantage that explosives would not be required. It would be safer, but by not using explosives it removes the element of acquiring explosive and blasting caps and would be simpler. A rock breaker hammer could be used.
There is also the possibility of combining cutting/sawing with chipping. A saw could be used to cut the boundaries of a channel and the ice within could be chipped/chiseled out.
However, I would seriously consider melting the ice and capturing the sublimating vapors using a suction device at the point of melting and storing them into a tank. Mars has an atmosphere and a suction device would work on Mars. A heated pad could be applied to the surface of the ice or holes could be drilled into the ice and heat probes inserted into them.