I have a line of landscape bricks (retaining wall type) that line a garden in the yard. I spend more time weed eating in those little v shaped spaces than on the rest of the yard. I'm thinking of filling in the gaps to make a straight line. What would be easiest yet long lasting - mortar or cement?

I would use mortar

Roundup is your friend.

I used cement. I was concerned that mortar would not endure well in a ground contact situation. That's not based on knowledge, only my feeling.

You will laugh at this, but I once used a caulk that never set hard and rubbed the face of it with mortar to give it that look. It worked perfectly and allowed for shifting. Now I use Roundup 365