In my opinion, cam sails have a fixed foil shape that remains pretty stable regardless of the wind. The center of force doesn't move around as much as no cam sail, which equals better control. I think this makes a difference, especially with larger sails (6.5 - 12.5). Also, the cam sails tend to have looser leaches with more twist at the top, which also helps with stability in gusty conditions. Once below 6.5, the need or benefits of a camed sail are greatly reduced. Some sail manufactures (Maui Sails) make 4 cam race sails (TR-5) down to 4.3. However, I doubt many are buying the smaller sizes other than the PWA slalom racers that may have to race in 35 knots. Even in those conditions, they use larger sails (5-7m). I think a no cam sail is easier to jibe, but it may give up a little bit in power coming out of the jibe. Regardless, I think a no cam sail is best below 6.5 in size.
There are many, many excellent no cam sails below 6.0m. I think you would be better off with one of them and they will be less expensive.
The jump from a 137L to a 90L board for you is pretty big. It's doable, but it may be pretty frustrating. I weigh 79 kg and you will find that uphauling a 90L board will be very, very difficult, which means that you must have your water starts perfected before getting on the smaller board. Same with jibes or you will be water starting on every jibe with the smaller board. Finding a board around 105 - 115 liters would be the best size to transition to your smaller board. You can uphaul a 105L board with some care. It won't sink if you have your feet in the right place, but a couple of inches off and one end or the other will sink. Wave action will also make it more challenging to uphaul.
The 90L board will need a solid 15 - 22 knots of wind to be fun with a sail around 6.0m. Your 4.9 sail will be fine in 20-25 knots on the 90L board. A 5.8 sail or still better, a 6.0 would be a good fit between your sails.
Hope this helps.