As the owner of a 6" f/6 Newtonian, I confirm what others have already said, you will not regret it. Under an urban or suburban sky it will not give you a great advantage over lower apertures, but under a dark sky you will notice those extra centimeters.
Newtonian telescopes, if they are well collimated, offer a great image quality in the center of the visual field. In your case, an f/8, the quality should cover a good part of the field. In addition, the CO is small and that impacts on a higher quality of the contrast of the details. For planetary observation I recommend you use eyepieces of a certain quality, which does not necessarily mean expensive. In my case, for example, I use orthoscopic ones (the Baader Classic), which are relatively inexpensive but of great quality.
You can see all the planets in the Solar System by appreciating the disk; Neptune and Mercury will be the most challenging because you will have to get close to 200x and you will need good seeing, and Mercury is always there, inserted in the glow of dusk or dawn. As for deep sky, you have at your disposal practically the entire Messier catalogue and many NGC and other catalogue objects. You just have to keep in mind that if you are looking for faint and diffuse objects, you will only be able to find them in dark skies.
Another field that you can also enjoy with your telescope is the observation of double stars, many of them with beautiful colour contrasts, such as gamma Andromedae or the so-called Albireo of winter (HJ3945, in Canis Majoris).
Here are some representations of how Jupiter and Saturn look with different telescope sizes; yours would correspond to the central representation.
