I had the same question when i bought my Flow hive mid 2019. I went for series 1 for the following reasons.
The series 2 has optional leveling feet. They are good for setting up and then their purpose is done with. I had some leveling feet from a washing machine that I screwed to the series 1 base.
Similarly, 2 has a spirit level. I have several in the shed. Series 1 has the correct angle built into the base so once the base is level everything above that is tilted correctly.
Series 2 has the tray, series 1 has the corflute slider. There are advantages to the tray, like being able to use oil to control SHB. I stuck narrow wooden slats, 3mm X 5mm, along the perimeter of the slider to make a simple tray. I use it in the bottom slot with a layer of diatomaceous earth on it. The bees can’t reach the stuff but they push the beetles through the mesh and they die on the tray. Each week or so I throw it in the bin with 10 or 20 beetles, and replenish the tray,
When I harvest the frames I empty the tray, turn it upside down and put it in the top slot. Any honey leak falls to the clean side of the tray and the bees can get to it. After a few days I go back to using it as a beetle death layer.
Series 2 has a window on each side of the super, series 1 has only one side window. I’d like windows on both sides, but that is about the only thing I really miss from series 2.
Series 2 also comes in red cedar that can be oiled to keep the timber look. That doesn’t matter to me, I’m happy to paint it anyway.
I think that’s everything I can think of.