[SpartanTau]

HAMMERHEAD GUNSHIP
Railgun Variant
Good:
- Best anti-tank weapon in the game with amazing range
- most versatile tank (can take out Land Raiders and Guardsmen)
- Heavy armor
- a Tau tank (skimmer moving fast, vehicle upgrades)
Bad:
- everyone hates railguns, especially when they are mounted on an almost indestructible skimmer
- secondary weapons don't really match main weapon
- expensive compared to other Tau tanks

Ion Cannon Variant
Good:
- MEQ killer, especially when combined with Burst Cannons
- better at taking out light to medium tanks than the Railgun
- Heavy armor
- a Tau tank (skimmer moving fast, vehicle upgrades)
- much less expensive than the Railhead
- secondary weapons more useful
Bad:
- is a Tau tank with Tau technology, which everyone hates
- more offensive/frontline nature can lead to an early destroyed result

Recommendations:
Usually, if you are tooling for a specific enemy (Ravenwing, Eldar), the Ion Cannon with Burst Cannons is the better choice. It puts out nine shots a turn and maintains all of the protection of being a Tau tank, plus enemies may ignore it for a turn as it doesn't have a Railgun on it. It is great for taking out marines as the Ion Cannon negates their armor save and wounds on 2+.
For a Take-All-Comers list, the Railgun is usually better. The main weapon is so versatile that at least two Railheads is considered required for a Tau army of at least 1000 points. Of course, it is much more expensive (in my 1500 points list, my three Hammerheads are a third of my points) and much more feared for it's destructibility.
For both variants, the SMS is usually a better choice, unless of course you are tooling up for a specific threat. When combined with a Markerlight source, both tanks become absolute killing machines, hitting (possibly) on 2+ and basically incinerating whatever they just hit.
There is a reason why everyone who plays against Tau expect these sharks, and why (almost) everyone who plays with Tau uses them.