Unless anything changed in the FAQ, taking an additional hand weapon means you don't get poison on either.
It kinda depends on if you think your Steg will be charging. If he will be charging, cav spear, if you do not think so, additional handwpn. If you are charging that is.
Why? It is not something like Obsidian Blades that stipulates handweapons, if you just have the poisoned attack special rule all your attacks, except magical weapon attacks, gets them. Poisoned Attacks Deadly toxins can turn an otherwise minor injury into a mortal wound. If a model with Poisoned Attacks rolls a natural 6 when making a roll To Hit, that hit will wound automatically. Unless otherwise stated, a model with this special rule may use it when making both shooting and combat attacks. Any spells cast by the model are unaffected, as are any attacks made with magic weapons.
Interesting, well I guess you're right then and yes an extra poison attack for a few points is a bargain
About the new post-FAQ Slann: If you want to stay in dispel range of enemy casters then you're most likely in range of their spells. A couple of fireballs and you have yourself a dead frog and ethreal is not going to help. Ethreal still important vs normal shooting though. If you don't use Slann for dispelling then a huge part of his strength is missing. I feel the real new tax for Slann might be both a unit of Temple Guard and the ethreal upgrade.
Becalming cogitation my help too. But yes temple guard is much more important now, especially for a non ethereal Slann.