I would actually recommend the regular .52 Gal over the Deco for Rainmaker, as its kit is simply more effective on a wider array of maps. Though Deco may be more similar to what you're used to, you'll find many situations where Seekers simply aren't that viable. And Ink Strike I would hardly ever recommend for Rainmaker because you're effectively giving the opponents free ink to charge their specials with, and its hardly ever useful for anything more than a quick Rainmaker Shield burst, which other specials can do in addition to more.
.52 Gal has Killer Wail and Splash Wall. Splash Wall is *the* defensive sub. You can stand behind the wall and get splats for a few seconds of immunity, and block key chokepoints. Your team mates can also benefit greatly from the walls. Its fantastic support/defense. And Killer Wail, in my opinion, was made for Rainmaker. Since you can see a marker for the location of the Rainmaker at all times, if your opponents are carrying it and you can't reach them, launch the Wail for an easy remote kill, stopping the push. Or burst the shield from a distance if no one's gotten it. People tend to cluster around the Rainmaker too, so it's not uncommon that you may get an extra splat or two in your trajectory. Especially useful for more linear stages like Arowana, Port, Hammerhead, or even Triggerfish, but I've had success with it on far less linear stages like Flounder Heights as well.
The Gal subclass as a whole are VERY solid shooters. Very well-rounded with few, if any, major drawbacks. So whatever you go with is sure to be quite effective. I think you're sure to find yourself winning more encounters with it than you were with the Aerospray. I just wanted to present my case for the standard .52 Gal.
