Well, I'm a huge fan of Russians as many of you knows :P So I will always say go for Russian if you can find one!
As always temperament is a matter of personality, we had some babies that were sweet from the first moment of their lives right out of the egg, some who were totally crazy (okay, it was only one baby that we hatched out of the whole clutch and she calmed after a few months) and some who were rather shy and "flighty" as a babies but are totally laid back as adults.
What I like the most about them are:
size - they are slightly longer than corns/kings but waay more bulky, so you "feel" them more but they are still small enough to be handled without problems
hardiness - to have sick russian you need to basically leave them totally alone for a few months... If you have food, water and are cleaning their tank they will be happy, healthy and totally hassle-free
eating - I never heard of russian that was refusing food without reason (they will sometimes! stop eating during breeding season and some prefer smaller prey items so if you move them to bigger prey they might refuse)
personality - of course like I said they are all different but in general even the more nervous hatchlings calm down with age and size... they are very active and like to observe whats going on, are incredibly inquisitive and love to explore, they will often try to climb on hands while the owner is changing the water or spot cleaning their tanks. As they are hardy species that thrive well in „normal” temps they can be handled for a longer time than tropical snakes. If startled their first response is to run, not to bite, but most of the times they are more curious than scared. Out of all the ones I had, seen or heard about only one was biting... and it was that crazy hatchling of ours that calmed after three months.
And of course I also like their looks, even if they are not as flashy as some other species out there
Ps. Parts of these reasons were from article I wrote so sorry if it looks weird if I start sounding more "serious" in these fragments
