In the end, you may choose Prismatic Laser > Photon Geyser depending if you like stronger attacks while sacrificing a turn - and which may turn up more useful in doubles, depending on your allies. But I think overall that Photon Geyser is a good move for Necrozma to have.

Pokemon prismatic lasermove

The 6H range of borescopes have a narrow cable just 5.8 mm in diameter. The different models vary in cable length, either 18" / 45 cm or 36" / 91 cm. The flexible nylon shaft will fit into holes as small as 6 mm diameter but it is not 'obedient' (it does not retain the last set position).The endoscope has an on/off switch which allows you to leave the light on without keeping your finger on the button. The handle design gives the greatest comfort for extended periods of use and ease of focusing.The lamp is an internal halogen bulb. The superior optical cable gives a high quality sharp image (7400 pixels) even at close ranges as low as a few centimeters. The cable is abrasive and water resistant with a bending radius of down to only 7 cm.Spare halogen lamp, angled mirror, and magnet accessories are included.

The 6H range of borescopes have a narrow cable just 5.8 mm in diameter. The different models vary in cable length, either 18" / 45 cm or 36" / 91 cm. The flexible nylon shaft will fit into holes as small as 6 mm diameter but it is not 'obedient' (it does not retain the last set position).The endoscope has an on/off switch which allows you to leave the light on without keeping your finger on the button. The handle design gives the greatest comfort for extended periods of use and ease of focusing.The lamp is an internal halogen bulb. The superior optical cable gives a high quality sharp image (7400 pixels) even at close ranges as low as a few centimeters. The cable is abrasive and water resistant with a bending radius of down to only 7 cm.Spare halogen lamp, angled mirror, and magnet accessories are included.

Prismatic Laseror Photon Geyser

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Pokemon prismatic laserwalkthrough

Cable diameter: 5.8 mmCable type: Flexible (non-obedient)Handle length and width: 160 x 130 mmConstruction: Ruggedised fuel and chemical resistant plasticField of view: 40 degreesOptimal min viewing distance: 2 cmPower supply: 6V lithium photo 223 battery (included)Light source: Halogen bulb

Also, Photon Geyser actually uses either Necrozma's Atk or SpAtk stat, depending on which is higher. Therefore, you are assured to have the highest amount of power dealt out of Necrozma (without considering the opponent's Def or SpDef). Since Necrozma has a much better SpAtk stat, you may favour choosing this stat to be it's primary attacking stat, and therefore have a powerful Psychic special move. But, you may play around with Necrozma's stats and turn Photon Geyser into a powerful physical move. So, you can adjust Necrozma to suit your battle style with Photon Geyser.

Photon Geyser is Necrozma's second highest psychic base-powered move, with the first being Prismatic Laser. However, with Prismatic Laser, the user will need to recharge on the second turn. Therefore, in two turns, you're actually comparing 200-base power vs. 160 base power - making Photon Geyser more useful in my opinion since it doesn't need a second turn to recharge.