Softwood and hardwood tops have different tonal charactoristics. Yes, softwoods tend to have more overtones and bring out a broader range of the tonal spectrum, but hardwoods tend to have more mids, a stronger fundemantal note, and often a lot of warmth. The science of the vibrations is useful information for understanding and describing these differences but ultimately tonal preference is subjective. I personally love hard wood tops. Both my ukes and my steel string guitar are solid mahogany. I would choose my hardwood top guitar over a softwood top guitar every time. They all have wonderful warm, woody, full mids that you can really feel, but still plenty of richness across the tonal spectrum. You should go with whichevers tonal charactoristics you like better. Again what sounds "better" is largly subjective and in the ear of the beholder. Maybe describe the tonal properties you like to the maker and see what they reccomend based on that description.
EDIT: Or as you say, if the subtle differneces in tonallity aren't that important to you, then yes, just go with a wood you think is unique and interesting and that the luthier still thinks will work well as a tonewood. The build matters more than the wood anyway, and since you are using a skilled luthier I'm sure they will be able to make just about any wood sound great. Personally, I always think it's cool when luthiers use a wood that isn't necessarily uncommon or exotic in general, but that is uncommon in luthery, like your London Plane instrument.