Modelling Trusses in Etabs or Staad 1

I don't know why either would give a different answer. Are you using p-delta or other iterative analysis in one?

If both models are identical they would be remarkably similar. I suspect you have modeled something differently.
Try end conditions, support conditions, load magnitude/direction, code requirements, etc.

Review your shear, moment, and deflection diagrams in both software packages.

This was drilled into my head by my senior and I try and drill into the heads of all our young engineers. The diagrams don't care what software was used to create them and will reveal real fast any wrong member or node fixities. This advice applies for a simply beam to a full building model all these software packages are very good at visualizing the results and some can even animate the deflection which can show you weak points in a frame model and soft stories in a building model.

Also of large importance with Staad and Etabs or any node based software are member unbraced lengths and how they are captured in the software. I know a beam with a node added at mid point but not segmented in Staad will have an unbraced length eqaul to the distance between the end nodes, likewise if it was segmented the unbraced length would be the distance from end node to the mid point node.

Is your etabs truss within a full building model where floor/roof diaphragms might be affecting the truss behaviour?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

I've never used Etabs.....but STADD will take into account elongation, shear deflection, etc that some software meant for nothing but truss analysis doesn't. That may explain some of the differences. Hard to say without seeing it.