Hi Adam
I have both books and I usually end up using both of them. There are advantages and disadvantages to both. And often I find myself digging into the Fungi Europaei volume as well.
The FNE book is much more comprehensive in terms of its species coverage and descriptive information. But it includes a lot of species that may not occur in the UK.
Geoffrey Kibby’s book clearly states which species are not yet known in Britain and you may get to an answer more quickly than with FNE, but sometimes there just isn’t quite enough detail to be certain of an identification (particularly for species which are new to you) and a second source with more detail can help.
FNE also includes the author’s thoughts on the identification of specimens in earlier publications covering Tricholoma (highlighting some of the challenges with this genera). It includes comments on Geoffrey Kibby’s book in respect of 5 of his illustrations on which they take a different view.
I couldn’t recommend one over the other because I find both are very useful.
Ken