As the other replies imply - it depends what you are looking for. Pages is wonderful but very restricted, like most of Apple's own software. You need to try them both to see what suits you. Nisus is great - the nearest to Word IMHO. Mellel, Scrivener and the others are somewhat specialist and require a lot of work to fit to your needs, if you like to automate, have sequential multi-layered lists, etc. Eg I write long documents with numbered paras, different types of headings, tables and figures with captions. Pages can't cross-reference, which is deal-breaker, and has irritating features such as inability to incorporate tabs into an automated style (eg make a style for heading or a para, and you have to opt-tab to insert the tab). Nisus is *very* customisable, including a comprehensive ability to associate keys with actions, and macros, whereas Pages can do neither of these. It does not take long to master styles in Nisus, and once you have you may conclude that it is the easiest of all WPs to automate styles in. Nisus has no graphics, whereas Pages has shapes, but you can use the tables menu to make text-boxes and lines, though this can be cumbersome and in some instances not possible (I cannot understand why Nisus does not deal with this). You can customise the 'palettes' also, though there could be some improvement there. It imports from Word far better than Pages, and seems to me (so far) to be the most suitable if you work in an environment that requires Word imports and exports. Another critically important feature is that Nisus is good with sections and you can insert easily (easier than Word, as with most things in Nisus) landscape sections - Pages cannot do this.
There are some things I have not tried yet, such as Bookends and Mathtype, but the general opinion seems to be that they work well enough - Mathtype is no good in Pages (or in Word for Mac), although I cannot verify this yet. Doing much of the above on Mellel is too hard, if possible at all (it won't do sequential multi-layered lists without a huge amount of workaround). A wonderful feature of Pages is the whole screen view, and Nisus cannot emulate that (though if you zoom and put a nice gutter colour you get near, but no WYSIWIG nor footnotes - Nisus, why can't you improve on this?). Nisus could well be the best WP for Mac, though as I said it depends on what you are looking for: it is certainly the best if, like me, you started on a Mac in 1980, migrated through different systems to MS-DOS and PCs, in Windows stuck though to Lotus Amipro, then Wordpro, and finally gave in to that horrible beast Word when it started to be able to do things that other WPs had done for years and the opposition went out of business, but then migrated back to Mac with Intel. In other words, if you customised Word, Nisus is probably for you, as the learning curve is not steep. There are many other things one can say, but I hope these comments help anyone - we should all do our best to develop diversity in the industry, which is too dominated by big players who produce bloated, expensive and useless software