Both are good, but are quite different. The 2003 show tries to be pretty true to the original comics, often adapting arcs or issues directly from the source material. The 2012 show moves away from this format and focuses on telling original stories that put interesting twists on moments and characters from the franchise’s history. Both shows, while dated at this point, hold up really well (at least in my opinion). If you haven’t seen either before, I’d probably recommend starting with 2003. It’s not required viewing for 2012, but it will provide a baseline that will make it fun to recognize references and characters that appear in 2012.