Age old question...Bad Battery or Bad Alternator???

I noticed today whie cruising my gauge was reading under 12V. I droive about 15 minutes / 10 miles and parked her at the store. After 90 minutes I went back home and my gauge still reading under 12v. I tested the battery and got a 12.20 reading. I tested the battery under load and no load @ 2000 rpms and got readings of 11.46 & 11.50. I went back later (about 30 minutes to test her again and she cranked twice and died. I charged her up and gott he slightly lower readings. I then parked her in the garage and got a reading of 11.95 on the battery. Note: Battery is 6 1/2 yrs old and the alternator is 10yrs old. Also, I always disconnect the neg. cable to the battery after every drive to avoid any unneccassry drains and as she only gets driven once a week. Any insight? Gonna take her to Pep Boys for a true battery test on Weds.