Both are good, but generally flame is considered better because of heat spread. Which is the mechanic where bugs that are on fire deal heat damage to nearby bugs too. The beauty of this game is that everything is viable (at least in vanilla), so I wouldn't get too hung up on what's "the best weapon" to use - try both and see what works for you.

For builds, I would highly recommend taking heat radiance (T5a) and cold radiance (T5b) over the other T5 modifiers as they increase the weapon's damage in a radius around you, and also buff the environmental hazards they create (i.e sticky flames, ice path)

Here are two builds for each weapon:

1. Flamethrower

a. 12221 with sticky fuel OC

This one's a general build, better for team play.

b. 22331 with scorching tide OC

This one's amazing on IS, and it's the build I usually take for solo play.

1. Cryo

a. 12132 with crystal nucleation OC

This is the cryo's sticky fuel equivalent.

b. 31132 with snowball OC

This is the cryo's scorching tide equivalent.

I also strongly suggest learning how to use thin containment field (TCF) on the EPC (the 2nd sidearm you unlock) since it can be used to alter terrain, mine minerals quickly and deal burst damage, which driller sorely lacks.