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It all depends on what kind of vehicles you are fighting, what their armour vaules are, and what makes up the rest of your army.

Vs AV 10-11 the waveserpent is king. Of coures I mean a standard scatterlaser setup here. There's only so much high volume medium strength firepower that light medium armour can take.
With the waveserpent's maneuverability this firepower is still pretty viable against many battletanks, as long as you can hit that side armour.

Vs AV >12 you've got warwalkers, or that flyers with 4 str 8 shots a turn. Alternatively there are always wraithknights, either with shooting their 2 str 10 shots, or assaulting.

It all comes down to what else you have in your list, and how well the anti tank choice compliments it.

I would certainly take double lance walkers. You already have wave serpents for anti infantry, so you don't need the scatter lasers in this regard, and as others have said, 2 lances > 1 TL-lance.

The problem with the fire prism is this: 1 shot.
Sure, it's AP 1, and if you manage to hit and get then pen roll you need, then you stand a decent chance of destroying a vehicle, however 1 shot at BS4 is still dicey.
However, for the cost of 1 fire prism with a holofield you can get 2 warwalkers with 2 bright lances each. These also have the lance special rule, and AP2 is still decent.

Here are some numbers for you, the chance of 1 prism shot, or 4 bright lances, at damaging a landraider:

1 prism shot: 0.666 hits per turn. Average of 0.333 pens, and 0.111 glances. Average hull points removed: 0.444. Total chance to destroy target: 16.7%

4 lance shots: 2.666 hits per turn. Average of 0.888 pens, and 0.444 glances. Average hull points removed: 1.333. Total chance to destroy target: 29.6%

I'm afraid the numbers do not get better for the fire prism against lighter targets either. So clearly, war walkers offer a better damage output vs vehicles.

Lets discuss durability now.
Fireprism with holofields (you did take them didn't you??) has a cover save of 4+ in the open if they move, and better if behind good cover. AV 12 is better than the war walkers but still nothing special. Can be put down with 1 destroyed or weapon destroyed result.
War walkers get a 5++ save all the time. 10 armour means you have to keep them back. Multiple vehicles make them more reliable against unlucky dice rolls. Can abuse Battle focus to shoot and then hide, or hide and then shoot.

Personally I prefer the walkers, as they are more efficient at the job you have planned for them. And with proper distancing, and target prioritization, you can keep them relatively safe.
Yes the prism has a bigger range, but with a 36 range and a 6+d6 fleet move the warwalkers should be in range of most things as well.

As a final note, warwalkers are more effective vs MC's than the prism. Which can count for a lot depending on your meta.

Ok sure. Against tau the prism would probably be better. But this is the first time you have mentioned tau. Against most other armies the war walkers would still perform better imo.

Tau have the advantage of non-LoS weapons, most other armies don't.

With that said, tau are only vicious up to range 30, your walkers could fire at 36 and then run back afterwards. You will be out of range of most of the return fire then.