The main advantage of an 8 inch telescope over a 5 inch one is when you take long exposure photographs. The NexStar mount isn’t good enough for exposures longer than around 30 seconds. The planets and the Moon will look the same in both telescopes unless you live in the desert away from city lights. Otherwise the maximum power is less than 300 due to the sky conditions and both telescopes can give you that.
Polar alignment is often needed for astrophotography. The NexStar 5SE has a built in wedge in the tripod which allows you to easily align the 5SE with the Earth’s axis of rotation. The 8SE does not have this option. If you want to make a time-lapse movie of Jupiter spinning around on it’s axis the 5SE is the better telescope. This video goes into more detail.
This telescope sold on eBay in August of 2011.
Here is Thunderf00t’s movie of Jupiter:
You can skip the first two minutes to get to the Jupiter animation.
Here is another YouTube video that shows why you want to be polar aligned for Jupiter animations:
Learn about the different firmware versions:
Here are links to some of the accessories you can buy for your telescope:
Case for NexStar 8SE
Case for NexStar 5SE
12 volt PowerTank
Celestron NexImage 5 webcam for telescopes
Nice quality eyepieces
Visit Cloudy Nights. They have a very active NexStar forum where you can get answers to any NexStar question, usually within just a few hours.