Looks like the abyssal plane has snakes on it too! (don't hit me)
Mariliths are of course best known for their six arms. This allows for a variety of fighting styles, from wielding one really big greataxe to six-weapon-fighting. Note that the latter will eat up a huge chunk of your wealth, so know what you're getting yourself into!
The demons have great ability scores. Strength and constitution are nearly 30, with all other scores at least 18. They're also Large (reach!), fast and can grapple and constrict people with their tail. The constriction has the added bonus of causing people to pass out if they fail a saving throw: too bad nearly everything can be immune to it at this level.
SLA's are a mixed bunch. Magic Weapon is completely useless at this level, Align Weapon duplicates an ability you innately possess, so it's only useful to buff allies' weaponry against a handful of specific enemies. Greater Teleport is useful, but not incredibly special at this level anymore. See Invisibility is completely useless as far as I can see, because you already have continuous True Seeing. Project Image may be potent, but I can't help but dislike it because of the short range and mandatory line of effect. Finally, there is Blade Barrier (solid damage), Telekinesis (versatile) and Unholy Aura (useful when fighting do-gooders).
Mariliths can summon a bunch of dretches, a hezrou or a nalfeshnee. They can also attempt to summon a glabrezu or marilith, but the chance of success will be greatly reduced.
What LA to put here? I tend towards something in the +1 - +3 range. Because a dip to get pounce in whatever class gives it is more-or-less mandatory on a melee character, and because I want to allow for at least one more level beyond said dip, I'm tending towards +2.