I have both artists in my Top 5, and both are extremely skilled at what they do. They both have impressive catalogs. With that being said: I place 2pac higher on my GOAT list than Common. I acknowledge Common's music and lyricism. Common's use of poetic convention & figurative language is above average to say the least. I credit Common for making some of the best hip hop love songs of the 90s, snatching LL's crown right off his head. I like Common's positive energy on tracks & subject matter. "I Used To Love H.E.R." is probably the greatest song in Hip Hop, and the theme of hip hop being a woman is still in use today. It's kind of weird, because when Common released that song Hip Hop was in it's golden age. Common is certainly worthy of GOAT status.
However, 2pac was/is just amazing. His songs always reflected the status quo of the times and the plight of the urban black male. Whereas Common's songs were very broad in delivery, Pac's was very detailed and personal, and conveyed the emotions of the characters he used. "Brenda's Got a Baby" is still one of the most moving songs in Hip Hop, and a lot of what he rapped about is comparable to the movie Precious from last year. "Part Time Mutha" & "Papa'z Song" were no exceptions to Pac's style and continued the trend vivid stories that evoked various feelings from the listeners. "Dear Mama" was the first song I heard of it's kind, and is still a trend in use today; Nas, Kanye, and even Lil B made songs for/about their mothers, and who set that trend? His use of voice altering equipment for songs like "Soulja Story" or "Soulja's Revenge" was the first i've heard of it's kind and a tactic used to create characters for his stories - something Common hasn't done, and also something artists like Nas have used. His music is timeless & his techniques for making said music are still in use today.
If Tupac is not high on a GOAT list for his musical accomplishments, then he should be included merely for his inspiration to Hip Hop. Many of the greats have bit his rhymes and his formula for commercial success & controversy. This is a good match up though. I like this.