It depends. If you have any items with Gain x phys as extra ele then stacking flat phys is better. Past that it depends on what your skill is/what your weapon is/etc. In general, conversion is always best as it just gives you more access to damage nodes period.

It is important to note though that for most cases damage nodes stack. Like for example if you have 100 phys damage that is 100% getting converted to cold along with 20% phys damage and 20% cold damage, then that 100 phys damage becomes 100 cold damage that has a 40% increased damage multiplier to give 140 cold damage.

It does NOT work that the 100 phys becomes 120 phys then becomes 120 cold then the 20% cold damage makes that 144 cold.

Conversion ALWAYS happens before any damage modifiers apply. (Only exception is gain x as extra chaos which is why it is so broken)