Let's talk about the Riptide.
Basics
It's T6 with 5 wounds and 2+ Save, 5+ invulnerable. Pretty survivable.
BS3 is the norm with Tau and it isn't any different with the XV104.
The Jet Pack allows you to Jump-Shoot-Jump. If you use the Nova-reactor to nova-charge the Jet Pack, you get to use 4D6 (!) for your jump in the assault phase. Great for getting out of reach.
Heavy Burst Cannon vs. Ion Accelerator
The HBC gives you 8 shots of S6 AP4 at 36" and if you use the Nova-reactor, it gets you 4 more shots and makes them rending... however, it also makes them Get hot in addition to the nova-charge's risk of overheating. 12 shots that get hot... hmmm, that's on average 2 shots that overheat and which you need to save against. (If you use "perfect" dice... using GW's dice,
that's an average of 3.48)
Equipping the IA gives you 3 60" S7 AP2 shots. Overcharge adds +1S and fires one Large Blast with Gets hot. Nova-charge is another +1S and makes the shot Ordnance. On this gun, the Nova-charge is usually not necessary. S8 gives you double strength instant death vs T4, S9 adds nothing to that. Everything up to T6 is wounded on 2+. I don't know a lot of stuff that's T7 or above (except Tyranid MCs with Iron Arm *grmbl grmbl*) and if it is, better to shoot it with the normal mode of the gun.
Conclusion: I'll take the Ion Accelerator. It gives me more flexibility and less chance of killing itself.
Secondary Armament
This all depends on what (secondary) role you want the Riptide to fill.
You want to slag some heavy armor? Take the Fusion Blaster.
Want some more dead heavy infantry? Grab some Plasma Rifles.
Need something to help blast light infantry from cover? Smart Missile System to the rescue!
Maybe it's best to give the Riptide something to help fill a certain role (or hole) in your army.
Just remember that the Riptide has BS3. It's not super accurate. These things are twin-linked and, to add to that, you can use the Nova reactor to shoot this weapon twice in a turn!
Conclusion: I usually face Nids or Guard on the battlefield, so I think I'll take the Plasma Rifles. This allows me to, again, be flexible. It hurts heavy infantry but also can (help) take down vehicles, especially when hit in the side or rear.
Two drones or not two drones?
At 25 points a piece, this is a pricey upgrade.
It gives you some added survivability in the form of 1 wound each with a 4+ invulnerable save and some more firepower with the missile pods mounted on them.
However, they also make the Riptide into a unit. This allows you to join up with independent characters which can give you interesting bonuses. Shadowsun gives the Riptide Stealth and Shrouded. This gets expensive sooner rather than later however.
Downside to being a unit is that the Riptide will need to take a leadership test if a drone is killed.
Conclusion: I'll spend my 50 points elsewhere.
Support Systems
If you take the drones, a Target Lock might prove useful. This allows you to fire the missiles at something and the Riptide's gun at something else.
The Velocity tracker is good for shooting at flyers, but for 20 points a bit on the expensive side.
The Stimulant Injector gives Feel no Pain. Do you want a super-survivable XV104? Do you have 35 points to spend? Get this!
Early warning override is also a viable upgrade if you plan on facing deep striking enemies!
Conclusion: I think I would take the Stimulant Injector, but only if I can't use the points somewhere else. The Riptide is already an expensive unit and you don't want to go overboard.
Example build
XV104 Riptide with Ion Accelerator for 185 points. Adding the Stim Injector bumps him to 220.
How do you field your Riptide?