The processor will make the bigger difference. That being said, the hard drive can be upgraded later, whereas the processor cannot. For this reason alone, go with the 2.53 i5 trim. A faster hard drive does impact battery life to some degree - but honestly, not by much, if even by any real notable level.
Mac mini (Mid 2011) 2.7 GHz i7, 8GB RAM, Crucial M4 256GB SSD + 500GB + 1TB FW800 OWC Mercury Elite Pro mini
iPhone 4S • iPod nano 8GB • Sound System Audio Engine A2 • Display UltraSharp U2412M 24"