When you attempt to merge accounts, the system will always select the account with the lower user ID to be the base user, because that is the older account.
We frequently tell people not to worry about this, because it really doesn't matter at all. The only thing that might be different is the public info such as display name and bio, but you can very easily change this. If you're concerned, you can also just copy these details so that they're the same on both profiles before you complete the merge (remember display names are not unique).
Past that, all your credentials, posts, comments, and everything else end up under one account, so there's nothing else attached to a particular account that really requires merging into a specific profile over another.