I got this DVD from the local library and was wondering which to watch first? I have never seen any B5 but I know that In the Beginning was made later but set before The Gathering so I'm not sure which to watch.
Different schools of thought
In The Beginning
is a prequel, intended to be watched after you see most of the Series unfold, my opinion is that's best, so, you don't inadvertantly spoil anything.
JMS' opinion is it shouldn't damage your viewing pleasure to watch In The Beginning
It is a rather fantastic Introduction to the Series, and a fantastic Movie, and since JMS says it won't hurt your enjoyment, it should be OK.
Definitely, on a rewatch, it is a Stellar First Episode, but, I still think (despite the Creator's opinion) it's best to watch it later on your first run through the Series.
It's definitely a movie that could help to more easily indoctrinate someone into B5 watching.