I have used a Browning Gold 10 gauge for many years. I use it predator calling, turkey and waterfowl. The Browning Gold is about a pound {or just a little over} lighter than the Remington SP-10. The Browning has an aluminum receiver, the Remington has a steel one. They are both very reliable shotguns. I didn't want to carry around the extra pound and a half of the Remington and I was a little concerned about feeling the extra recoil, but honestly I believe the Browning kicks less. Maybe it's the recoil pad or the stock dynamics, but it feels less to me.
As far as the advantage of a 10ga. over a 3.5" 12 ga. you have to consider what the 3.5 12 is for. it was developed as a way to get enough payload weight in a 12 gauge with steel shot. It does get the job of that done, but at a cost of the best possible pattern for that shot charge.
The 10 gauge is a whole other world of patterns and shooting long range, especially with lead shot. One thing I want to be clear on...when it comes to waterfowl vs. predator ammo...you will feel a lot more recoil with lead shot. A 2 1/2 ounce charge of lead will let you know it on both ends, but it's not so bad that it is not going to be worth the extra power and range on predators. I definitely reccomend you get a 10 ga. and I also suggest you reload for it.