We've got a Woosh XF07 front on ours. Excellent for what we need, but more power and lower speed torque would be better. (We were warned by Woosh at the time, but there was no effective other choice for 700c wheels available from Woosh or elsewhere).
Rear hub would be better in some ways, probably better overall. However we've never had significant wheel slip on the front; and sometimes on slight uphill wet grass had slip on the rear and been kept going by the motor pulling at the front. Different weight distribution on the tandem reduces the downsides of front hub.
I think one of Woosh's 48v kit as mentioned above with low speed torque would be very good (if available). I certainly recommend Woosh, advice before buying very good, they took tandem measurements and made up the cables accordingly, and quick helpful advice as needed (luckily very little needed) after fitting.
