A slider may be more convenient, but it will add extra weight, which may or may not be a issue?

It's very easy to use your packs to adjust trim. On a old forum someone mentioned having a rope attached to pack bottom loop. You can use your paddle to push weight into bow or stern, rope to pull packs to center when applicable.

I prefer a web seat for a couple of reasons. If you want to kneel , its more comfortable and when it rains the bucket can collect water unless it has drainage holes?
As you mentioned you have a canoe seat, why spend the money for another bucket canoe seat.  M_M has it right, try the bucket and slider if you can then you can make a decision based on whats best for you ;)