I've had both. From my experience, these are the various pros and cons:

Live-in Nanny:
Pros - hits the ground running, doesn't need lots of hand-holding and help at the beginning
- Usually has much more experience in childcare and genuinely loves working with children
- You can actually meet them and interview them in person because they aren't subject to visa rules
- They aren't subject to any time limits or visa restrictions
- You can negotiate whatever contract terms you want

Cons - generally more expensive (though all of the money goes to the nanny and not to an agency as well, as with au pairs)
- Hard to put this kindly, but it can be hard to find a normal, high-quality, experienced person who wants a live-in nanny job. Some of the best candidates actually don't want a live-in position but will accept it as they see it as a benefit you are getting, and don't want to take less pay for it. Presumably what you are looking for is for someone who sees it as mutually beneficial to have free accommodation, in exchange for a lower amount of pay than a live-out position.

Au pair:
Pros - more flexible - they can be scheduled to work when you need them, so you can have them do 2 hours in the morning and then 3 in the afternoon, etc, as well as weekends.
Pros - the cultural exchange part, they often want to be part of the family and join in with activities, and that can be great
Pros - cheaper, even though you have steep agency fees

Cons - all interviewing has to be done on skype, etc and you need to be really clear about your expectations, and really understand their motivations and expectations.
- many au pairs aren't particularly experienced in (or even interested in) childcare, and many won't have done it full time before, so even with the best of intentions the au pair may find it much harder, and there will be a long transition period at the beginning when she learns everything and you feel comfortable that she can do the job
- can't take care of babies under 3 months
- have a limit of 45 hours a week. In reality, most au pairs work far less than this, and you may have a very resentful au pair if you schedule her for the full 45 hours, as she'll soon see that most of her friends aren't doing that.

Anonymous wrote:How old are your kids, OP? Au pairs don’t care for newborns.

No one-on-one care of infants under 3 months. And they can’t even live in the same house with a child under 2 unless they have more verifiable experience with infants and toddlers.

How old are your kids, OP? Au pairs don’t care for newborns.

Au pairs can be good if you have elementary-school children and want a “fun big sister” type to look after them. I personally would not use them for any younger than 4yo. Nannies are fine for any age but the higher-quality ones tend to want to live out so you will probably find housekeeper-nanny types for live-in (which may very well suit your needs, again if you have older kids).

All of the kids o know who have AU pairs are total nightmares behavior wise. Maybe that’s why they got au pairs in the first place, I don’t know.