You can't compare a Formula and Serenity in light wind. These are in 2 distinct leagues IMHO.
With your 58 kg and 10 m2 sail, I guess you can plane in 6-7 knots of wind with a Formula. Below that, you will be shlogging at no more than 5 knots hull speed (and it's becoming rather dull with time).
The Serenity is way faster than a Formula in shlogging mode, especially in 4-7 knots of wind, or even less (not tested yet). Nothing dull when gliding in such low wind, quite stocked in fact...! What I've found with my 10.6 sail in 4 knots of wind is that when the Serenity starts to move forward, the pressure on the sail starts to increase as well and I do need to hook in with the harness. Kind of with increasing apparent wind speed, the Serenity accelerates and glides effortlessly over flat water at quite an impressive speed (I wouldn't be surprised to reach a hull speed at twice the true wind speed or more). I never experienced that when schlogging with a conventional board. This is unique to the Serenity so far.
In winds at and above 7 knots, I'll be curious to compare side by side the planing HS105 with the gliding Serenity. So far with the same 10.6 sail, the top range of Serenity is about 8 knots with its 70 cm fin, higher with its Shallow 41 cm fin (cannot say if board is the limiting factor as yet), but with HS105 and a 40 cm fin, 12 knots is the top range I can go, the sail and not the board being the limiting factor with my light weight. I feel I really don't need a Formula or an Apollo to fill the gap between a Serenity and a HS105 for my light weight in light wind. Both nicely complement each other.
Try a Serenity if you can in winds below 7 knots with your 10 m2 sail, I bet you'll be stocked as I'm.