The crowd levels for Fantasmic will depend when you are going. There are some Fall events (like the Food & Wine festival) that tend to increase the crowd levels at the parks. If you are planning on going in September, there should not be a long line to get in to Fantasmic. Keep in mind that this had been the case when they were showing Fantasmic every night.
As for the Fantasmic dining package, they are booking up very quickly. I talked to a CM when making ADRs last week and they told me that due to the limited showings of Fantasmic (now only a couple of times per week instead of nightly) the dining packages are going fast.
You can do the Fantasmic package at Hollywood & Vine, Mama Melrose, or the Hollywood Brown Derby. I've eaten at Hollywood & Vine and Mama Melrose and I prefer Hollywood & Vine.
If you do decide to book the Fantasmic dinner package, be aware that the dinner reservations are much earlier than typical dinner times. Most are usually booked before 5:00pm. They do this to allow you time to eat a leisurely meal and still be able to arrive early for Fantasmic.
I've also eaten at Sci-Fi, and still prefer Hollywood & Vine. Sci-Fi's menu was a bit limited for me. My children however love Sci-Fi. It's among their favorite restaurants in all of WDW.
Hope this helped.