Totally up to yourself. Neither of those have anywhere near the number of features vBShout 5.0 though.
Personally i would recommend trying to upgrade to PHP v5, if it causes problems, it's very simple to roll back to v4, and you'll have to update to PHP v5 eventually. Most places no longer support v4 - including the PHP developers, and indeed the new versions of vBulletin.
Your call though.
Iain
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