I’ll start by saying that there is quite a bit of press about why there is a general lack of support by theme developers for post formats. (heres one example)
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There in lies the first deciding factor – if you have a theme which doesn’t support post formats (most dont) then you can’t start this way.
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Second point, from a development perspective if you want to write a query to just find posts of the format Aside that is easy, but to find just standard posts you have to do a query to exclude post formats aside, quotes, etc, etc, etc). So it’s a pain to code for
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If you wanted to have different views / layouts / themes for each post format type, then that’s far easier to do with WordPress network and much harder with post formats
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Finally … all post formats (asides, standard, quote, etc) are included in your WordPress loop so by default they will get included in any RSS feeds, custom WP queries which means you end up un-necessarily, having to re-code ‘a lot’ to exclude post formats from places where you don’t want them to show.
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Which reminds me, WordPress MU Sitewide tags will allow you to agregate all content from your sites into one master blog. This is what I do if you have a look at http://wordpress.damien.co and http://damien.co