Margaret pros -
Cool nicknames (Meg, Maggie, Margo)
Cooler than Elizabeth in that there are SO many Elizabeths
Won't be one of several in her school
Cons:
The full name is a bit ugly and reminds me of an angry middle aged woman from the 1960s with her hair in rollers smoking a cigarette.
Elizabeth pros:
Cool nicknames (Eliza, Betty, Betsy)
Full name sounds fine
Cons:
A bit boring
Will probably be one of many Elizabeth/Elizas not to mention all the other El names that blur into one
Personally i know it is hugely unpopular on Mumsnet but I'd choose one of the nicknames for full name: maybe Margo or Betty