Yeah, for some reason the lower keys of some alloy whistles tend to clog more, at least in my experience. However, it might not be a problem with the whistles in general, but corrosion, and the toothpaste trick might solve that.
In the Goldie/Chieftain/etc sort of family of whistles, I’d recommend a Lofgren, if that’s what you’d like. They are cheaper than either, I think, and frankly, I like them more. I don’t have one in Bb, but I have a Goldie in soft blowing low Eb, and in the past I had some that were in-between medium/hard blowing in low D, low F, low G, Bb, and C and I have the alloy Lofgrens in low G and soprano D (waiting on an A), and I like them much much better. I can’t imagine the Bb being disappointing if both the low G and soprano D are so spectacular. Oh, and unlike chieftains, they are actually made by one person (who is also a good whistler), not a factory.
If you like Chiffy sort of Gen tone, the Sindt is really in my opinion the best choice for that, but it’s been mentioned, and it’s not easily obtainable, and I don’t think worth a kidney, either. There are other great whistles out there, too.
I got one of the relatively new Dixon “Trad” whistles in Alto A, and I like it very much, especially for the cheap price I got it for secondhand. It’s quite similar to a Gen Bb, maybe too similar, if you are wanting something quite different. The tone is less clear than a Gen, just a tiny tad bit more smokey, but it plays very similarly, and the sound is what I would consider in the same “genre” of sound as a Gen Bb. They make them in Bb, also, but I’ve only tried the A. I don’t know if the Bb and A use the same bore size, or not. If they do, they might differ in playability between each other. But, if it’s like the A, then I’d say it’s a good choice if you like Gens, as I think they are similar in many ways, and it fits your description.
The Syn is absolutely fantastic in B and C. I got a set of Bb, B, C, and D. The D seems to me to take a hard push for that key, and the Bb seems to take a very light push. However, if a lower key like Bb or A is boughten, I heard that you can get a head that takes more push, so this factor is not so present. I think they are one of the most underrated whistles around, at least in B and C they are. I always avoided trying them, thinking they might not be so good, but I got them and they are one of my favourite whistles. I kind of see them as a sort of combination between some of the qualities of a Burke and Sindt in some ways. The tone is full and what I would consider on the clear side (at least concerning complexity, smokeyness, raspiness, etc), yet at the same time chiffy, and sort of has that playful blippy quality of a sindt/dixon trad/freeman.. I think it has a good mix of the more “traditional” whistle qualities, yet with a strong full tone like some of the other high end whistles around. Not sure if you’d like it or not, but I am a huge fan of them. I generally like the Gen/Freeman/Sindt/Dixon Trad type of whistles (if I were to choose one overall sound), and while the Syn is different from that genre, I still loved it. It mixed the qualities I like out of those whistles, with the fullness I like in others. So, if your tastes are like mine, you might like it as well. I can at least say the B and C are my favourite whistles in those keys, more than Sindt, Burke, Freeman, etc, even. That’s saying a lot, too, as I like all of those very much.