In terms of portabiility I think it is the same.
In terms of ease of use IMHO I think gtkmm is easier. Everything is handle in C++ terms. Eg a GtkButton is class named Gtk::Button which has all the functions that you use with a GtkButton*. Eg let's say you have Gtk::Button called my_button and a GtkButton* called my_gtk_button. To show the widget in gtkmm you just do a my_button.show(). In Gtk you do something like this gtk_widget_show(GTK_WIDGET(my_gtk_button)) that's ugly and needs more typing. Also with Gtkmm you don't need to play around with pointers(in most cases) and so you don't need to worry that much about unrefing widgets.
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