An 8-inch aperture will certainly outperform a 5-inch, on the planets or anything else. However, aperture is only one consideration. When I first started observing I could not see much fine detail on the planets, even with 8.5-inch and larger apertures. When I moved up to a 14-inch I still could not see the kind of details sketched by Paul Doherty et. al. So, I switched to photography. After a few more years experience I realised that the eye and brain become trained to see finer detail the more you observe. Then I realised how crucial accurate
collimation was. I also realised that the planets rarely look really good unless you observe a lot and catch the good seeing. Observing planets on a laptop screen, when imaging, is a revelation. The details distort every sixtieth of a second with the limb jittering by arcseconds! Then, on maybe 1 night in 20, planets look like a Damian Peach image, when the jetstream is absent and the weather is misty, or even foggy! However, unless the telescope has cooled to the night air and is precisely collimated you still won’t get great views. Moving up from a 5-inch to an 8-inch will not automatically give you good planetary views. So much depends on the state of our atmosphere and how often you observe. On this latter point some sort of user-friendly observatory, that can be set up in minutes, will make you observe 10 times more often, because you can just ‘nip out’ and check the seeing. For planets you do not need a computer control. Despite the hype, low-cost computer controlled telescopes can be more hassle than you need. Planets are easy to find, so a low-hassle Dobsonian is all that is needed… No need for star-alignment, cables and pressing buttons…just push the Dob and look through the finder….
Deep Sky objects are obviously easier to find with ‘Go To’, but star-hopping is simple for the brightest objects.
So, to summarise, moving up to an 8-inch will not give you spectacular planetary views unless you observe a lot, train your eyes and brain, and keep the telescope collimated and cooled to the night air. The bigger the better though, so it may be worth thinking about a Dob. You can get a 12-inch Skywatcher Go To Dob for the same price as a Celestron 8SE, but you’d want to build a small run-off shed to house it, or put it on wheels. Deep Sky objects would look far brighter in a 12-inch! One final thought….Mars, Jupiter and Saturn are horribly low down at the moment so whatever telescope you get you have a long wait until they are nice and high again.
And the best views are usually obtained when planets are high up.