Are McMurtry's modern novels as good as his historical western fiction?
It’s difficult to predict whether you will enjoy his modern novels as much as his historical ones without knowing more about you. Both sets of neovels have been equally appreciated by critics, and both deal with western themes, so in a general sense they can be described as equally good. But the important question here isn’t some abstract sense of goodness, but what books you’d enjoy reading.
A way to decide whether you would enjoy them is to ask yourself what you enjoy about the historical works. If it’s the author’s style and approach, you probably would enjoy works set in other periods by the same author. If it’s the geographical setting, you’d probably enjoy most western novels. If your real interest is western history, you’d probably enjoy reading both fiction and non-fiction about the historical west by a variety of authors.