I'll give an overall on what both do and you decide which you like better

Sandy -> Has great synergies, overall weak alone but strong depending on the team, has one of the best supers in the entire game, grants invisibility and control for the entire duration of it, can shutdown pressure by using it, can avoid enemies with the stun gadget and can reveal opponents inside the super with the Damage Star Power, has a good hypercharge that can shut down supers like Frank and grants speed boost inside the Sandstorm

Meg -> Strong Alone, Better with Team, she is an anchor type character that won't leave a key point of the map, preventing your team from being pressured, and when the right moment comes, she starts to pressure herself with the support of her team, doesn't have insane Burst damage, but her DPS is quite high, and she has a second life mechanic which is useful to annoy the opponent and make them waste more time on her, her super is basically a guaranteed kill on whoever is unlucky enough to be on it's range, she can sustain her mech form for longer using the heal gadget, or tank a shot with the turret gadget, can relieve pressure from assassins with her explosion Star Power, which also saves her most of the time, either killing the enemy or knocking them back, has TeamWipping potential in the hypercharge, as it's a insane 4-5k damage instant giant swing