Pros of in-home daycare: one on one attention, easier to bond with one person which will mean your daughter will build trust and be happier and more comfortable in general, immediate response to needs, cheaper, most are more flexible with time and special requests [like the homemade food and breastmilk]
Pros of a daycare center: reliable, structured, accountability [with more people around it is unlikely your child would ever be abused- but it has happened], good daycare centers incorporate teaching activities, daily reading time, socialization, and balanced meals [one less thing to worry about]. They may have their hands full, but they will feed her when she is hungry, and put her to bed when she is tired, and they are required to change diapers every 2 hours.
I personally would not have someone watch her only for the summer. That's a lot of change in a short period of time. Think of switching daycares like switching schools for older children. Switching schools 2 or 3 times in one year means your child will have to adjust each time, and the instability could hurt her more in the long run. Especially if you get a summer sitter that she really likes and bonds with.
18 month-olds are definitely not able to be reasoned with! I think she would have an even harder time as she gets older. Her crying and tantrums and separation anxiety will only be more intense at 18 months.
My opinion is, start her in daycare now, or find another long-term nanny [not just for the summer].
My son has been at a Kindercare center since he was 8 months old [he is 15 months old now] and has done wonderfully. BUT....and here is the *BIG* But......He has also been sick ever since he started. He has literally not gotten better all winter. He gets sick, they send him home, I send him back when he is well enough to attend, and then he gets sick again...over and over. He has gotten everything...pink eye, ear infections- he eventually had tubes put in, high fevers, he has had a runny nose all winter, bad coughs, pneumonia, etc. Daycare centers are filled with germs and every kind of cold or sickness you can imagine. So if you send her to a center, expect that she will be sick for the next year or so. After that year she will have probably built up her immune system, but it is a huge hassle in the meantime. I thought about pulling my son out so that he wouldn't get sick anymore, but everyone told me "keep him in, because he will either be sick now, or when he starts school".
So that's what I think, but as other people have said, you should go with your gut and do what you think is best for your daughter