I personally say that a broadside is stronger since it is twin-linked and has a longer range.
The advantage the zoanthropes have over the broadsides though is their invulnerable save. and also the fact that they can now be thrown in a pod and deep strike anywhere on the board. Unfortunately their attacks are psychic attacks though and they might end up hurting themselves.
So between the two I would have to say that the broadside is stronger, but the zoanthrope has better survivability.
the Broadsides can target 2 seperate units from T1 but aren't that great at anti-infantry and are pretty slow even w/ASS. Their range means they are shooting from the get go. Their defense is improved w/Shield Drones and generally getting cover but they rely on the rest of the Tau army (i.e. Piranhas/Kroot) for their major defense in keeping opponents away.
Zoanthropes are arguably more likely to destroy higher AV vehicles due to Lance but require a spore to get into range early and quickly (which also means hive commander Tyrants). However, Zoanthropes have two extra dice rolls to get through in psychic tests (unlikely to fail) and psychic defense (40-50% failure) but at the same time if Spored correctly, can often get shots without cover. Zoanthropes have better anti-infantry ability with an AP3 blast but because of their short-ranged nature, they are much more vulnerable to taking hits and are more of a one-hit wonder (yes even with 3+ invuls).
In the context of their codex, outside of close combat, Zoanthropes are the best answer for high AVs for a Tyranid army. Whilst T-Fexes can damage AV14, Zoans do it well. However, with the emergence of psychic defenses so strongly and their reliance on a Spore and thus a Hive COmmander (x2) Tyrant, they are often used in a specific army. You'll often find Hive Guard are more useful for their incresed effectiveness against AV10-12. Broadisdes benefit a lot more from their codex. Whilst there are more anti-tank options besides Broadsides, sticking 2 S10 AP1 shots into seperate tanks each turn is very nice. Add in the ability for Tau to strip cover & their layered defenses and Broadsides are an invaluable tool.
Therefore, in context Broadsides are better and more important as they can split fire and pop transports relatively easily for Crisis Suits to munch on. Whilst Zoanthropes are Tyranid's only real answer to AV14, Hive Guard are much better anti-transport platforms and it's a prt of the game Tyranids have to deal with currently. Add in the psychic defenses... In isolation, it's close as they both offer extremely good anti-tank. Without psychic defense I'd lean towards the Zoanthropes but overall I'd say Broadsides are better due to their consistency and range.
Also remember, Zoanthropes are T4 and very vulnerable to getting insta-killed. Broadsides are too, but they can outrange most anything, sport 2+ saves and have drones to suck up extra wounds. A brood of zoanthropes is equivalent to MEQs in terms of durability against small arms, which is not all that tough really.
Unless you have a Podding or Reserve army, Hive Guard are a better buy.
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