In many ways they were fairly similar but there are numerous differences. I am a Germanic pagan (asatru) and know a little about Celtic, but very little. I know the Celts believed the head co contained the soul, and they would carry the preserved heads of their ancestors in ritual and their enemies in war, sometimes even throwing heads at their enemies.
the Celts were a much older culture from what we know. Their culture used to occupy most of Europe in Germany, Spain, parts of northern Italy and France. They had many "tri"gods, gods that had 3 aspects but were always one. This is a reason the Irish converted very willingly to Catholicism.
the early Celts and Germans did not seem that different except in language and mythology, along with art. Celtic art is often symmetrical and circular, based on spirals and complex patterns. Germanic art however is often asymmetrical and angular. It is quite primitive and barbaric looking compared to roman and even Celtic art but there is an oddly alluring quality to it, it is strange and abstract but ancient and powerful. Just looking at runes seems to emanate a sort of power.
the Celts also seemed a little more organized, at least in Gaul and Iberia, but the British Celts were as savage and warlike as the Germans, st least according to the Romans. The other Celts however had chain mail, albeit rare even among elites, before any other people. They also had a king ship system a little more diplomatic than the Germans.
their mythologies and cultures in their most explored aspects are very different, but to an outsider or initiated they appear to be just a palette swap. Even the Romans confused the belgae tribes for either Celtic or German, as their languages started to mix along the Rhine borders.
just feel and think, meditate on which names appeal to you the most. Wotan or lugh. Explore your ancestry as much as you can.