The visigoths and Ostrogoths were both confederations of pre-migration gothic tribes (eg the Thervingii and Groethungii), that agglomerated and reorganized into larger political groups while travelling west. The Visigoths’ formation is older, better attested, survived longer, and overall I would say their impact on history was greater, so if forced to pick between the two for a leader or UA, I would choose the Visigoths. I think you could attempt to portray both groups as a sort of lump, but it wouldn’t be precise. We also have relatively little to go on re: pre-migration Goths, just archeological evidence of their migrations south from Gothiscandza, shown by the spread of stone rings from there, a smattering of mentions in Roman and Byzantine sources, and the tyrfing cycle, a mythic origin of the Thervingi Goths which survived as a Norse legend.
The thing that is really interesting about the Goths is that their language is gone and left no descendants, but it is also the oldest Germanic language we have records of and the only example we have of an eastern branch Germanic language. We have almost completely reconstructed the language, because we have a well-preserved Bible translated from Greek into Gothic (the famous Wulfilas bible), so the Gothic language is a rather singular historic artifact.
Good leader choices would be Alaric I, Theodoric I, or Liuvigild
Their UA should manipulate either some sort of permanent policy bonus, allowing you to steal other civs’ policies if you capture their capital, or gain combat bonuses against civs with more cultural influence over you. If there is a migration mechanic, it could leverage your diasporic community for unit production or recruitment.
The one weakness with a Gothic civ is they don’t have many remarkable or singular unique unit or building choices. As a group that left their greatest mark during a time of upheaval an migration they didn’t leave much cultural or military innovations that might distinguish them, and the things that did distinguish them were more often their quick adoption and integration of conquered people and institutions. The gothic kingdoms in Spain existed for centuries, but there isn’t a single gothic loanword in Spanish, for instance.
For UUs, the Visigoth’s use of Groethungi heavy cavalry against the Romans at Adrianople, and against the Huns at the Catalaunian plains was rather remarkable. The aforementioned stone circles or stone ships from their putative homelands at the mouth of the Vistula river could be used as a unique improvement.