First of all: sorry for the late reply — I was on a vacation for the last two weeks.
Thanks for the nice words about Starling! I'm happy to hear that you already released several games with it! 🙂
As for Swift: I must admit I haven't done much with it yet, besides porting over the sample projects. So there might be some problems I just haven't run into yet. Performance-wise, though, you don't have to worry: yes, Swift is ARC, but if you use Sparrow from Swift, its internal code will still use MRC. Thus, the performance will be great.
Event for an Objective-C game, I'd recommend using ARC. It just makes your live much much easier. The really performance critical stuff is done inside Sparrow — and that's using MRC for you.
You're right, the tool support of Sparrow is great — especially since it shares the same file formats as Starling. That's definitely an advantage!
The advantage of Sprite Kit is that its Xcode integration is better, and it has integrated physics (if you need those). The disadvantage is that you have to learn a new API.
The advantages of Sparrow are the ones mentioned above; the main disadvantage is that — I must admit that — the Sparrow community is much smaller than that of SpriteKit. Many users seem to have moved over to SpriteKit or a cross-platform solution like Starling.
I hope that information helps, Oaken! 🙂
I wish you all the best for your game!