They're both very reliable. I haven't read anything by Johnston, but her non-IP "young adult" novels are well regarded, and she knows her way around IP, having written four Star Wars novels.
Johnson (no t) is the big news, in my opinion. She has five published Forgotten Realms novels under her belt already, including some really good ones. And her book will be the first Forgotten Realms novel since 2016 that doesn't feature Drizzt and isn't aimed at kids. That alone is very exciting news to those of us who like that sort of thing.