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Is the Aegis worth taking for 50?

I mostly play a 1200 point Tau force and I find with Marines VS Fire Warriors so much comes down to who gets to shoot first...

The 4+ Cover save would be a bonus when I am getting hammered but I am also worried about bogging down a unit that potentially could be needed further down the battlefield later.

A 4+ cover is nice, remember, dead marines cause no wounds.

Aegis is a fantastic investment for 50pts. Tacticals sadly (imo) are not. Shove devs, tanks and sniper scouts behind there.
Tbf an aegis does not benefit marines as much as lower save armies (still more than adequately benefits them for its points mind).
Take 10 bolter marines for 140pts. For the same points you can get 28 bolter henchmen.
For a shoot out slug fest lets look at the advantages marines have in comparison:
-BS 4 over BS 3
-T4 over T3
-3+ over 5+
-ATSKNF
An ageis helps lessen the latter two advantages to a massive extent. It gives the henchmen a better save against almost every weapon in the game (minus ignores cover and barrage with the hole on the wrong side of the ageis) whilst only benefitting the marines (with a worse than normal save) against a much smaller percentage of weapons.
It keeps more men alive, thus preventing as many leadership tests, hence less of a need for ATSKNF.
Still advise you invest in it if you are going for a long distance slug fest or want to use it as cover to close in (you can put it anywhere in your half not just in your deployment zone). But I would advise against a long distance slug fest against tau anyway.