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sorry let me rephrase that.
the canon you're looking at has USM, but people use that as a catch-all for the USM found in the higher end L-lenses etc. the USM on such lenses are "ring-type usm", which allows you to manually focus even if you're in AF. unfortunately, the canon you're looking at does not have this type of USM (I think the other one is micro-motor USM or something), so if you're in AF mode, you cannot override the focus manually.
It still has a MF/AF switch, which is what you were asking about.
in my experience, the canon lens focuses faster than the sigma/tamron equivalents, but it isn't as good as ring-type USM lenses.
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