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I'm a tester by nature, so I would suggest doing both and seeing what performs better! You could accomplish this by using the Random Sample flow step with 50% going to Engagement A, and 50% going to Engagement B.
In terms of moving between streams, it's nothing too difficult, assuming you aren't deploying the emails within a stream at different times (to account for time zones). What you would need is a smart campaign for each stream you are working with, which fires weekly about an hour before the second email is to be sent. If the lead has not opened the email, then move them to the second stream. If they have, do nothing. This assumes all streams deploy at the same time.
This means you give each lead the maximum possible time to open and engage with the email, and move them around appropriately. If you are running many thousands of leads through these engagement programs, I would suggest waiting longer than an hour to ensure there is time for Marketo to process them all.
Hope this helps!