They are separate and different sets of XTensions, although some of their functions are duplicated, so you would not be wise to have both versions of those XTensions loaded together. Obviously Quark's Xpert Tools Pro is provided free, but QX-Tools Pro is provided by a third-party developer, onOne Software, and a free 30-day demo version is available from their website for you to compare features.
Britsmith
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iMac • 3.06Ghz • 4Gb RAM • OS 10.5 • QuarkXPress 8.1
PC WinXP • QuarkXPress 7.31