To what extent are they different?
SNAP, ArcGIS or QGIS differently handle the visual contrast enhancement (smallest and largest value which is used for the color ramp).
To go sure, you can compare the image statistics in the metadata of both programs and check if the minimum and maximum values are the same between the different programs.
Some discussions on this:
- Exporting product to GIS with pixel data
- Importing RTC S-1 into QGIS
- SNAP vs QGIS images
- Wierd values around my colored image from SNAP to QGIS
Another thing is that conversion to GeoTiff might change the data type (e.g. 8 bit integer 0-255).