Ignore the processor speed thing between the IIi and the Mini s. One is Intel and one is TI so it's irrelevant (bit like the AMD/Intel thing) + the Mini S CPU is believe it or not a Dual Core CPU! one core is dedicated to the phone application which means the XDA actually works as a proper phone now
I had a XDAIIs for a year or so and have found the Mini wipes the floor with it, not just in performance but useabilty to.
Only bad point is the ROM on the UK spec Mini S currently only supports 802.11b WLAN, although the card itself is 802.11g so I'd imagine next ROM upgrade O2 release will resolve this.
Small features like the dedicated "Comm Manager" button for quick access to Bluetooth and WLAN and Mobile phone style softkeys on the front make it the best pocket PDA/phone I've used so far by a long margin.
Used to get a day battery life out of my IIs if I was lucky, get a week out of my Mini S