I’ve a couple of Primus ones. I’ve no reason to use anything else they are brilliant. Irrespective of make all I’ll say is you get what you pay for in my humble opinion.Yep, the best out there for heat retention and slim form factor. The 900ml one for all day and the 500ml version for a morning or evening outing.
The Thermos spec sheets show these as being top of the range with 24 hour heat retention - others such as the King series are only good for 18 hours.