The biggest difference between the Romans and the barbarians is the overall division of the barbarian tribes. Rome was a very centralized power which had the ability to levy taxes and thereby supply better arms and equipment to its soldiers. By the time of the late republic, it is believed that Roman soldiers were supplied armor at public expense. This means that a higher quality of armor, better than what the poorer Celts had, would be worn by the Romans. The aristocratic Celts were equipped similarly to Roman soldiers. They wore a chainmail shirt and a sturdy helmet (the Romans, in fact, based several helmet designs on Celtic helmets) and carried a good sword, though one that was quite long and often unwieldy, as opposed to the Roman combination of pilum and gladius. Poorer Celts wore a helmet and carried a sword and maybe a shield, and occasionally fought naked. This indicates a great deal of individualism in the Celtic and Germanic warriors, which means that personal prowess was absolutely a big deal. Classical authors also talk a great deal about the Celts being this way. I would recommend the works of Diodorus Siculus as well as Caesar for more information on the Celtic barbarians and Tacitus' Germania for info on the Germanic barbarians. Along with this, Adrian Goldsworthy wrote a biography a few years back on Caesar which discusses some of these things very well, and most assuredly in more detail than I can here. His book is called "Caesar: Life of a Colossus". Goldsworthy is a military historian, an Oxford educated one, no less. Warfare in the Classical World by Warry is also an excellent source on this. Warry is very well read on this topic, and presents his evidence in an excellent and entertaining way. His book is full of pictures and statistics on battles. A quick google search of Celtic arms and armor would also be good for additional context. Google images shows a lot of excellent museum pieces, many exceptionally well preserved. This includes spear heads, swords, shields, and armor.