I've got a low roof,.regular wheel base Transit and it works for me. I carry everything I might need and small parts, etc.
I think it really depends on the type of installs you do and where those are.
Do you have to deal with parking garages and downtown parking much?
I think the medium roof, regular wheel base Transit is about perfect.
I'd love a taller roof but my last van was also a low roof so I'm used to it. I also like to drive through the car wash weekly and park the van at home (gated driveway ) but don't want to push my luck with people complaining by seeing the van over the fence.