I went with the 550b so I could slow things way down when I want to. It is harder to do that with the 650. I even work up loads on the 550b one round at a time. Digital scales and calipers are alright. You will find they are not accurate in cold weather or when the batteries start running low. Analog doesn't have that problem. I do like the ease of using a digital. I would prefer to have an analog backup for both when possible. When measuring things that can change your day from good to bad, it doesn't hurt to have a backup plan.
When given a choice I would rather switch to a slower method rather than stopping everything and waiting for new equipment to arrive. I also just like seeing if one measuring device gets the same reading as another one will.