Is there a overhead using line by line option or wasted disk space?
There is a very recent topic about that in the mailing
list, look for "Time series
data model and tombstones". I believe DuyHai answer your
question there with more details :).
Yes, if you know the TTL in advance, and it is fixed, you
might want to go with the table option instead of adding the
TTL in each insert. Also you might want consider using TWCS
Here are some blogposts my coworkers recently wrote about
TWCS, it might be useful:
The Last Pickle - Apache Cassandra Consulting