Should I keep the same URL structure or redirects through htaccess are preferred?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  Walter Mrowczynski III: Following. I have a similar issue except mine was built in php-nuke between 7 and 13 years ago and I`m still getting hits going to url`s like modules.php?name=Forums
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  Alan Bleiweiss: Since you are rebuilding from the ground up, it may be the time to fix the flat URL structure. If there is a need for extensive URL organization, that would require proper topical organization planning. If only a few products, then domain/products/productname should be sufficient. Or domain/shop/productname could also work.
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  Aashish Singh Bisht: Currently, they have about 210 products with the structure store.domain.com/parent-category/child-category/product-name.html. Some products are not in any category, so they don`t have parent or child category names in the URL. The main website is http and the store site on the sub domain has SSL installed - https. I saw some product names with underscores instead of hyphens. Some products with TM in product names, the TM is also there in the URLs too. Can it get more complicated than this? pheww If the previous developers had created a sub-directory for the store instead of creating a sub-domain, they could have used the SSL for the whole website. I guess I will start with extracting/generating the proper sitemap first and see what all other URL structures are there. They want to add 30, 000+ products after I complete the redesign. Once a proper structure is set, then it shouldn`t be a problem. Please do let me know if I should take care of something specific in such cases, like if something is usually missed in such scenarios and we realize it later.
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  Dave Elliott: Also in my past life as a dev I really hate aspx as a thing. Get them well away from dot net nuke or whatever they are using.
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  Alan Bleiweiss: Aashish "They want to add 30, 000+ products..." That right there is all I need to know. It will be CRITICAL to get one, standard system and methodology, and URL hierarchical standard in place from the beginning. Every product needs to be assigned to a proper category/subcategory silo path, or if there is a valid way to refine that to category/subcategory/sub-subcategory/ then that is what the URL organization needs to follow. Any individual products that are not found in that silo path need to be assigned to it. If they claim "these products don`t fit in a typical silo path", then they need to rethink their category/subcategory model, or, at the very least, they need to come up with an alternate model for just those - such as "other products" or "miscellaneous", though I do my best to encourage clients to not go that route. If they have products they want to assign to multiple paths, they need to decide which single path is the most relevant, and assign it to that path. They can link to that product URL from other category sections, yet need to have only one consistent URL for each product, to avoid duplicate content problems.
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  Aashish Singh Bisht: These are some good points Alan specially about the products that the client think don`t fit into any category. I was tracking their backlinks and found out that the store was built using ASP.Net earlier and not Magento. At that time the store resided in a shop sub-directory domainname.com/shop and later they created the shop on the sub-domain shop.domainname.com. So there is a lot of link juice going to domainname.com/shop which gets redirected to shop.domainname.com. This is good. They have less direct backlinks to the subdomain store. I will probably summarize after the projects is completed about what all I did for this redesign into bullet points for others to read.