I bought new boots last year. I looked at pretty much everything....Fox, Sidi, Gaerne, Alpinestars, MSR, etc. I ended up focusing in very quickly on the Sidi, Gaerne, and the two Alpinestars you mention. For some reason, the Sidi just didn't fit me right, so I forgot about that one. It was a very nice boot though. The Gaerne (like the one Larocco wears) was really nice too, but I decided to get some Alpinestars instead.
I tried on both the Vector and the Tech 6's. The Vector was without a doubt the most comfortable motocross boot that I've ever put on. However, I wasn't too keen on the ankle support even when new and I had read that they soften up considerably and become less supportive. So, I ended up going with Tech 6's. They weren't quite as comfortable when brand new, but they broke in very quickly (2-3 rides). Great soles. You can't even feel those hard impacts from landing hard. I struggled a bit at first to feel the pegs, but quickly the feeling came back. The boots hold up very well in my opinion. Actually, the only marks that I've put on them have been when my foot has slipped off of the kickstarter and nailed my right footpeg. It's put little marks in the inner plastic then.
All-in-all, I love my Tech 6's. However, there are a lot of good boots out there. If the Sidis fit well, I'd consider them. Take a look at the high-end Gaerne too. That's a nice boot.