Brambor might be onto something here. I used to wear those spandex pants with padded chamois when riding my road bike long distances and I think the riding position also kept by derriere from getting sore; most of my weight was expressed through my legs and arms when pedaling at high speed. The upright cruiser bikes (like the Metro) tend to position more weight on your butt and this gets transferred to your back and neck.
Consider the following images referenced from
this post at RevoBikeReviews.com. Body position does play into aerodynamics and stuff but it also impacts your field of view when riding and the physical strain put onto different body parts. The further forward you lean the less weight is placed onto your rear. Also, seat height plays a role and how actively you're pedaling (which is usually lower on ebikes). If you're pushing hard with your feet then your rear isn't going to have to support the entire weight of your body the way it does on a soft comfy sofa... or your wife's cruiser. Fun fact, Electra patented their "flat foot" design that actually brings the bottom bracket forward from the seat post tube and provides an even more bench-like seating position that's meant to be relaxing. You might notice that from 2013 to 2014 Pedego had to stop using this design on their own cruisers due to patent rights.
Justin, you might not have the same issue with the E3 Dash because it's delivers a more aggressive forward-leaning body position. Still, proper riding attire can help and you can always consider one of the
Thudbuster seat post shocks as well. You might only need the short travel ST. Just remember, with the cruiser bike you'll still be experiencing pressure from below as you bob up and down... it's just not going to be as abrasive with a suspension seat post. For me, this mostly helped the back and neck but then again, my behind never really feels that sore... so far
