Powertrains should be different which will play right into how they want to divide the two models. BMW alone has a major advantage since for all we know they'll pull from the 4-series, much like the M-Coupe and 3-series back in the day
Overall once it meets the power requirements we'll be happy.
With a lot of new Lexus and Toyota models coming to market so soon, odds are this engine was developed with multiple models in mind. Regardless of what it is, we're in the clear.
If some are saying it will be one or the other, then they might be wrong because odds are they will offer a couple engine options. Just one powertrain might not do it. That's if they want to aim for mass appeal on all levels, price being right at the top of that.