There are several questions that need answering. Size of parts, there abbility to absorb the heat. Then what equipment you have to heat the job. Sif bronze is cheaper and strong but needs more heat than silver solder. Silver solder has little gap filling abbility where as Sif bronze will gap fill and give a good fillet. Noel.

Knowing what the build is for would help.

Copper tube isn’t strong, so presumably this isn’t a weight bearing construction.  Could be a sculpture, in which case bolts or self-tapping screws are easier than hot joints.   They’re no good if the pipe has to be waterproof though!

A combination of steel-sheet and copper tube suggests a heat exchanger to me, which is trickier due to heat and wet and might have to be strong too.  Steel touching Copper corrodes when the joint gets damp.  More bovver if the construction gets hot because Copper expands a fair bit more than steel.   Hot water copper pipes are usually carried in clips and have generous bends so the pipe can move as necessary without breaking anything when the temperature changes.

Silver Solder is more expensive than Brazing but less skill is needed.  Both require a blowlamp powerful enough to get the joint up to temperature before the flux fails.   Sifbronze always often needs Oxyacetylene.   Plumbers Solder is weak, but could be acceptable.   Or even plastic ties or Superglue.

Always go for the engineering solution that meets the requirement most cheaply, but no cheaper!

Dave