I'd opt for the 23", the more screen real estate the better IMHO unless it just can/'t be swung in the budget. They they probably have different screen now (or not) when checking them out at the Apple Store the 23" seemed brighter when cranking them up to max (blinding actually).
And don't buy a Dell, sure they're cheaper but that's like saying Walmart is a quality retailer, they're not. Why support a sh***y computer company with a soulless pr*ck of a owner who repeatedly bashes Jobs while totally completely idolizing him not to mention copies in many cases. Some of us have morals and principles and are, quite frankly, not that cheap.