I believe you're right about the features, RC.
Periscope will probably win on Twitter (makes sense).
I imagine Facebook is watching Meerkat closely and deciding whether to buy them, or build their own.
I just tried Periscope.
Thoughts:
Good app experience.
(Young) people are going to be filming everything. At least initially. They will over-share.
People are going to hit cellular data caps very quickly.
People may quickly become annoyed at casters... There might be bans at public places like Google Glass.
Someone was filming their dinner from a restaurant.
Their friend (eating) looked annoyed.
Someone else was filming his drink being poured at a pub.
The bartender looked annoyed.