If you know that you've got backup, then FDC is a really good option. My DS is in FDC one day a week, and with my mum one and a half days. I know that he is getting consistent care in both - given that he has only just started to self settle to sleep since starting FDC I would not be comfortable putting him in long day care. The only downside to FDC is if you are wholly reliant on it, eg: my FDC carer is just about to go on maternity leave for a few months, or if her or her own child are sick then she can't work. But in your case (as in mine) this is not an issue as you have family on hand when needed.

I've found that with one day per week my DS has taken longer to adjust to being in day care than what children who are there more regularly usually do, another reason why FDC is a better option for him as the carer can devote more time to ensuring he is happy. He is also not particularly social and gets a bit overwhelmed by a large number of kids so the smaller group size is much more conducive to him developing social skills.