I think that it would be better to lose all your old memories than to lose the ability to make new memories. I would not want to be living forever in the past. I would want to focus what is going on in my life right now because right now is the place where life is happening.
They both would be terrible but if I had to choose one I would much rather lose all my memories than be able to create new ones later on in life rather than be forced to give up any new memories. I think I can guess what life was like from pictures and writings.
Our memories are what sustain us; some people love to think back to happier times, which is called nostalgia. So we really do want to remember the past, or at least parts of it, since it can be a blessing to us. Additionally, it's very important to be able to retain new memories, so that we can constantly be learning about the world around us. I'm not sure why anyone would want to lose all of their memories, unless they had grown up in a very difficult situation.
I don't think losing all of your memory is preferable to losing the ability to retain new memory. Losing all of your memories would be losing your past. You wouldn't even know who you were, or who your family is. Either scenario would be devastating but to lose all of your memories is like losing your identity.
I believe it would be preferable to retain all of your memories, if I had to make a choice. By retaining all of your old memories, you would still be able to remember friends and family members. One of the hardest things to adjust to with Alzheimer's sufferers is when they no longer recognize their family or the people who have loved them.