Ok, some math.
1/2 hit, 1/4 wound, 1/20 wounds per point, AP5
2 shots, 1 hit, 1/3 wound, 1/18 wound per point, AP6.
1/2 hit, 2/6 wound, 2/42 wound per point, AP5.
1/2 hit, 1/6 wound, 1/30 wounds per point, AP5
2 shots, 1 hit, 1/6 wound, 1/36 wound per point, AP6.
1/2 hit, 1/4 wound, 1/28 wound per point, AP5.
Obviously against T3 the devgaunt is the shootiest variant, but it is only AP6. Therefore the other two species will usually do better. Against T4 the fleshgaunt is the shootiest.
But the spinegaunt is always close to the shootiest unit, and it has greater numbers, so casualties are less severe and with these vast numbers it is better in close combat.
Additionally, big bug swarms look better, there is a psycho effect