I would go with the oxblood since you already have a black one. I think oxblood goes great with several colors - brown, cream, black, white, denim. Even though it is a dark color, I would even carry it in summer with a light colored shirt and accessorize with a necklace or bracelet with maroon/burgundy/ruby accents.
I agree with PBL - definitely the oxblood if you already have a black sofia. The oxblood is a very versatile colour and the sofia is a very versatile bag. In fact, I've a black sofia myself and am eyeing another one in oxblood! :)
I love the Sofia! Any Sofia in any color is awesome, but I think I'd pass on oxblood if I had black. Oxblood I associate more with fall/winter, and black fills that need. If I were in the market for a second Sofia, and I had black, I'd go with a more summery color. Possibly a lighter neutral like caramel or tan.
I have an Oxblood hobo and really love the color. Have you seen the Hermes color Prune? It is very similar to that. A burgundy with a drop of brown mixed in. I bought my Oxblood Hobo in the summer and really do not have an issue with wearing it year round. After reading this thread, I need to pull it out and wear it some more.
As far as getting multiples of a bag in different colors...nothing wrong with that, either. There are some BV ladies who have double digits of certain styles. If you love it, you love it.