In a similar situation recently we went to both and showed up late to the second one. The second party was a closer friend, but the first party was more fun for my son (at one of those places with the inflatables)
It depends on the age, my son is 5 and at that age, birthday parties seem to be a event of power 'i'm not going to invite you to my birthday party if you're mean to me' gets thrown around a lot. So for him being at both parties was more important than being there for the whole event.
At my son's party two boys showed up for the last 15 minutes, just so they could 'be' at the party even though they had conflicting parties.
You need to do what is right for you and your daughter - the party moms are not going to care. The only thing you might want to consider is if the first mom has paid for a group price and it won't matter who comes or not, or if you show up to play part of a game she will have to pay more than she would if you didn't show up, that might make a difference.