Redirecting smaller successful site to domain of new site - SEO impacts

Question

Our client has 2 websites, each on their own domain, let's call them A.example and B.example, both interesting to the same demographic.

A.example is a small but very successful single purpose site with plenty of inbound links from the entire industry, including competitors to B.example. It has virtually no competition itself and is the go-to source of its kind in the region.

B.example is a newcomer, both its scope as well as its actual size are much bigger. It has strong pre-existing competition. The owner of the two sites is thinking about merging them both under the B.example domain, as either A.B.example or B.example/A (while leaving a redirect on A.example), basically to bring SEO benefits to B.example (branding and everything else will also merge).

What would be more appropriate, A.B.example or B.example/A? Are there any SEO dangers to this, would simply linking from A.example to B.example be better? Or should we suggest the other way around, B.A.example (though this could prove to be a hard sell to marketing, but as long as we're talking technically)?

Answer 1

This looks to me like the classic question of subdomains versus subfolders for SEO.

When you use subdomains, Google's algorithm has to make a judgement call of whether to treat each subdomain as a separate website for ranking purposes, or coalesce the ranking juice into one logical website, or somewhere in between. When you use subfolders, that's pretty much the strongest signal you can send to Google that these resources are all part of the same website and should accrue E-A-T together.

In practice, people often see that when they switch from a subdomain setup to a subfolder setup, they get a ranking boost.

I've used the subfolder strategy for my own website (posting a bunch of somewhat-related resources as subfolders on the same domain) and I can attest that it works well for SEO.