Home DIY

I would go with Soudal, who are a dedicated sealant manufacturer.
The Dow brands, in the UK, are handled by Geocel, who had a sealant facility in Plymouth bitd, and may be manufacturing under license.
Mapei are relatively new to sealants, filling a gap in their offering to wholesalers/pro-retailers who prefer to consolidate to as few suppliers as possible. I suspect the Mapei tube sealants are made by a large EU manufacturer, maybe even Soudal.

Performance will not vary very much as it is hard to integrate much biocide into a clear sealant, both physically and legislatively.

Ive tried it all, cheap or expensive and feel a lot of it is the same. although CT1 have a new thing called BT1 which is apparently mould resistant, ive used it on a couple jobs recently. Not been there long enough yet to see if any mould comes on it.