I own the standard SX3 with a 28" barrel and have spent lot of range time with the Maxus. I was fortunate enough to spend a day with the design engineer for both of those guns and shot the Maxus at a "day at the range" at the SHOT Show when it was first introduced.
They are both very fine guns. They share the Invector+ barrel but the Maxus has the longer forcing cone and a few other innovations not found on the SX3.
Both guns are exceptionally light and easy to carry. They are also "soft shooting" but both will let you know that you're shooting a full power turkey load. A 3" shell is plenty to shoot in either one.
If I had to choose one, I'd pick the one that felt better in my hands and came to my shoulder best.
I don't think you'd go wrong with either and I would not let the longer barrel be a factor in you decision. As light and maneuverable as these guns are, I've not found the longer barrels to get in the way.
Thanks,
Clark