You might pay some heed to any "reliability guarantee," too, under which the software company promises to pay for any mistakes (such as in calculation) its program makes. This is not so important if your taxes are simple and small; the mistakes probably won't happen and can't be sizable. TaxCut and TurboTax advertise such guarantees.
Least important of all when considering tax software are newfangled "electronic filing" and "1040PC" features. Electronic filing lets you pay $10 to $20 to zip your return off via modem or mailed disk. That way, the IRS gets its faster, you can get your refund much sooner. The 1040PC form is a simplified printout of your tax return, which includes only line numbers and dollar amounts, not all the words and rules on the traditional tax form. The IRS accepts such forms and can process them faster. Both electronic and 1040PC filing are fascinating, but not make-or-break features.