This is just my opinion, but if you are getting more than core I'd certainly go for manticore. It seems to have unparalleled mobility and killing power that would offer a different experience than other units. Full disclosure, I ended up penny pinching and opting out of both because the stuff I am getting is already probably too much game, and I haven't seen the models irl or playtested any of them.
Echinda's Children expansion seems to have a lot of overlap with other abilities in the game. While those monsters would play differently, I don't feel they really 'add' a lot of strategic depth over what you'd already have in the core+stretch goals. The basilisk petrify ability is found on at least 3 other units. Fox is really elusive, but so are several other units, etc. As far as I can tell main draws for Echinda's Children expansion are the models and familiarity.
The other factor is that Manticore is kickstarter exclusive while Children may not be. Odds are if you want the Children later you can get them (that's my plan).