I like both Don H's and Glenn's vocals equally, but I think they are each suited to differing styles (note this is all very, very, very broadly speaking and there are exceptions!) - Glenn pulls off the more 'romantic' songs like PEF, Lyin' Eyes, NKIT etc really well and I think his softer, almost more delicate voice fits perfectly. Don's is I think more edgier, rougher almost, so he pulls off the hard-rocking likes of Witchy Woman, OTB and LITFL better. Of course, if you bear in mind Out of Control, Already Gone, BOML and Hollywood Waltz this all goes out of the window...
It's hard to explain!