Which Rhyhorn to evovle?

The one with higher CP?

If you want a defender then you want the highest CP...so the 2nd one.
If you want an attacker then you may as well go for the highest attack stat for a Rhydon, as the stamina will make no difference....so the 2nd one.

Make your decision based on whether you want an attacker or defender.

save both and get more candies :)

Rhydon is not great attacker or defender and the value is that it can have high max CP and thus rank high in gyms . So higher attack IV choice would be better , which is the second one .