Rather than talk about my opinion, let me tell you my experiences:
I’ve bought 12 brass gens over the last six years. Of those, 2 were in tune. Many of them are so horribly out of tune that they are the whistles I let my 1 year old son bang around, since he always wants a whistle when I’m practicing. I figure, if he destroys them, I’ve lost nothing.
During that same time, I bought 3 sweetones. All 3 were in tune. Three, because I broke one (back-pocket graveyard) and so bought 2 more–1 for backup. This a few years ago, and my sweetones (one unpainted) have not rusted, so I’m not sure what those other guys are doing to their whistles. 
I’ve very recently started hearing the claim that Sweetones ‘sound like a toy’, without any explanation as to what that means exactly. I own a Trophy brand toy tinwhistle (at $1.95!), and it doesn’t sound anything like the sweetone, though the ‘toy’ is more in tune than 6 of my brass generations. My own experience is that at the Irish sessions I play at, the sound of my sweetone has been complimented twice.
When I first started playing whistles, I had mini-WhOA going..I’d bought clarke traditionals, a shaw, feadogs, and waltons, all in search of the ‘perfect’ whistle. When I bought a sweetone, as still a relative beginner, I was so pleased with it’s sound and playability that it stopped my WhOA for a good two years. I went from sweetones to ‘high-end’ instruments such as silkstones and thin weasels.
Also, I’ve also bought about 4 different nickel generations in various keys. They seem to be better in tune right out of the box than the brass ones, though they do feel 'slippery’ish in the hands. So, if you have your heart set on a set of gens, I’d personally suggest nickel, as your chances of getting good ones seems to be better.
Greg
