Over the weekend I rode a 100l, 65l and 45l with a takoon xs940 and swapping between a reptile bora v2 4.2 and 5.0 to maintain similar wing power in the changing, gusty lake conditions (we had +- 15-25kts).
The 100l is a 2yo chubby custom (4'9 x 28") at 5.8kg. The smaller two are DIY boards from last summer (picture below). 65l is a freestyle wing board with a more traditional boxy shape (4'9 x 24", left) that has an indestructible carbon pvc construction with heavy duty foil box at 5.1kg. I build the 45l (5'0 x 20", right) as a light, fiberglass + carbon reinforcement at 3.5kg, prone board but it seemed appropriate to give it a spin with the wing as well for the purpose of comparison. With a winter wetsuit I probably weigh around 95kg. Here are my findings:
The sinker boards seem to have a relatively similar low end, i.e. +-3 knots more than the floater. Essentially, you need wing power to get to your feet. Once on your feet, it's easy to take off. This is surprising since the 45l sinks much more, but it is so slippery.
In the air the 45l was fractionally better in all regards compared to the 65l: slightly better at touchdowns, a bit more nimble,. The 45l outperformed the freestyle board by a fair amount when pumping in the swell (likely because it's 25% slimmer). The 45l was however a bit less forgiving. I felt no difference in (swing)weight. In fact, I'd make the 45l heavier because it feels less stiff when pushing for speed than the 65l.
Overall, I'd be happy to have either one of the smaller boards in a 2 board quiver. It's not that much of a difference imo