Hi @francesca,
I’m not familiar with these, but from the Micron Gemini product page it seems to be quite a new product, which is in the same body as the Gemini 720im (so mounting and space requirements should be similar). I also wasn’t able to find an actual product page for the 720im (the link for it just redirects to general products), so perhaps the Micron Gemini is its replacement? The Micron Gemini is apparently very new (as in, it was announced this week).
From a forum search of “tritech” it seems that “Tritech Micron” has been used to refer to the Micron (Scanning) Sonar (which in 2015 costed ~$15,000), and apparently the Micron name has now spread to being used for a family of small Tritech devices.
I would expect a multi-beam sonar to be more expensive than a scanning sonar, but don’t know by how much. Other community members may have a better idea, but it’s likely easiest to just ask Tritech directly.