I'm running 2.3 (Gingerbread) on an Xperia mini, and Sony Ericsson recently announced the availability to upgrade to 4.x (Ice Cream Sandwich). My understanding is that ICS is geared more towards tablets and large-screen phones. Would the Xperia mini with its small 3" screen benefit from this upgrade, or would ICS features such as action bars negatively affect usability?
If you are getting ICS update from device manufacturer, its Ok to upgrade. It'll be usable on small screen because you'll not get bare vanilla ROM.. Manufacturers put their own skins over it.
So, if Sony Ericsson is pushing upgrade to Xperia Mini, their custom skin would take care of usability.
Other than this, you will get performance boost with ICS upgrade. There're other benefits, too. So, I'd recommend you to upgrade. Best of all, would you like to be out-of-date?