It's somewhat contentious, as neither is commonly used.
Back in 7th edition, the Maleceptor was without much doubt the worst monstrous creature in the game. Expensive and defensively weak, with its gimmick being to drain your army of psychic dice for a power that had about as much chance to hurt itself via perils as it did to hurt an enemy.
It still has a bit of a bad reputation from back then.
These days it's still not a great unit. But you're not actively hamstringing yourself by taking one anymore. It's actually now one of the toughest monsters tyranids can take. The 4++ invul save giving it the same defensive stats as a hive tyrant.
It's nowhere near as good as a hive tyrant offensively though, which is why it's not seen often. But it's still capable of sprinkling out a few mortal wounds.
The Toxicrene is also not used much, as it's just another close combat monster with a gimmick, in an army packed with gimmicky combat monsters. The main gimmick in its case being the short ranged shooting attack that it can fire into or out of combat.
The main downside to the toxicrene is its degradation. As it loses wounds it loses 3 combat stats, WS, S, and A, which is pretty bad. There aren't many combat monsters/vehicles in the game that degrade in a full 3 combat stats.
In its top damage bracket though, it's moderately punchy for a nid monster.
Defensively it's a T7 12 wound monster with no invul save, so don't expect it to last long once anti-tank guns start looking its way.
Were I forced to choose between them, I'd probably go for the Maleceptor. Due to it being a bit more survivable against the kind of firepower that gets thrown around.
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