I have a '74 SF Princeton Reverb and the first thing I did was replace the stock 10" Oxford speaker, since it flubs out easily, doesn't handle medium/high gain drive pedals whatsoever (blurting/wooly saturation).
I went back and forth between an 10" Eminence Legend 1058 and Copperhead; in the end I settled for the Legend, since it had a bit more bass, sounded fuller/wider and more balanced. But, looking back, to retain more of the country/rockabilly possibilities, the Copperhead might've been the better choice to remain closer to its origins, while being able to handle drive pedals better.
If I were in the market for the amp itself, I would go for the '68 SF Custom version. Has a Celestion, IIRC.