In most cases, the 7AH battery will suffice. Just do not discharge to deeply. The typical SLA gell cell will be quickly ruined (very short life) if discharged to deeply. Always recharge as soon as the voltage drops below 12 volts under a normal load. It is also recommended to never charge any SLA battery at greater than 0.1C (700 milliamperes for the 7AH unit) else you will again severely shorten it's lifespan. You can get some good info on the care of Pb batteries on Red's site
http://www.hangtimes.com/redsbatteryclinic.html
The information on this site can save you many a dollar as well as frustration.