Oh Gee... That is a tough one. I think, in the end, I would choose what could create the most meaningful name for my child.
Things I would consider:
Lydia:
is biblical and a follower of
Jesusrecognizable and easy to put in combos
"L" names are big right now so it may be semi popular or blend in with more popular names
means "from
Lydia", which is not really significant
Dahlia:
is a perky and cheerful flower
I have a lot of family with summer birthdays and Dahlias are summer flowers
"D" isn't a popular letter right now so the name is less likely to be popular and over used any time soon.
can be spelled
Dalia and then has more significant meanings...
In the end, I would probably choose
Dahlia.