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Both are theoretically compatible with gates drive, but the nexus option may comes with it (based on their website pictures, but no reference to gates). I think that LvH now only sell eBullitt's with the option of 11-speed alfine or 5-speed version and have discontinued the 8-speed alfine. The 5 speed comes with belt drive, the 11speed alfine with a chain. You'd need a new motor unit chainring etc to convert the alfine, plus no autogear shifting and the durability issues.
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I have an EBullitt with e6100 drive. It's unstoppable and I put two kiddos (3 year and 1 year old) in it. I can fit one large shopping bag in it with the kiddos fairly easily. Knees bent.
I've very often loaded it up with 200 lbs of cargo. In a few isolated hair-raising occasions I even had 300 lbs of cargo. I wouldn't recommend that. Not including my own weight. So over 450 lbs. Not recommended, but worked.
I got a premade box from Splendid Cycles in the States. It converts from a kid / grocery carrier to a flat bed in seconds. Holde Box. I love it.
My ebullitt from @BrickMan has the 8 speed alfine. To be honest with the assist on the ebikes, a 5 speed nexus would have been better, fewer shifts, more reliable and belt drive. The hub is specifically designs for the steps system loads. The fewer shifting option is also really helpful if you use it in auto. I don't see the huge benefit of giving up the di2 integration for the rohloff.
Another positive for the IGH is it can autoshift down when stopped to your designated starting gear. For me this is gear 2.