Pairs are not necessarily from the same clutch (depends on how many pairs the breeder has that lay). Coxi tend to be a little less expensive (but you'll need to wait till the 2005 clutches hatch to find them). Coxi are more likely to eat pink mice from the get go. I've been working with my rhinos to get them to consistently eat f/t pink mice and it's been a challenge to move one from fish to mice.
I would rate coxi at medium difficulty and rhinos at experienced difficulty. I've been looking for a female coxi to pair up with my male 2004 for about 5 months and haven't found one, so good luck
till the new clutches hatch. Pairs will run between $800 and $1000.00. Also, be aware, that though they may start on pinks, they will move up to adult mice as they reach adult size. My coxi is more defensive than my rhinos, and will musk or attempt to strike on occasion. Not a big deal with small snakes, but it does make them more of a challenge to work with.
Check out some care sheets on both species if you haven't already. Both require lower than standard N.A. rat snake temps and higher humidity. Also, I would say that both are less tolerant of frequent handling, if you want to keep them feeding regularly. If you want a "pet" snake to handle frequently, neither really fits the bill, though they are both beautiful snakes.

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Lora