www.hostelworld.com
Hostels ain't what they used to be....
Lots of new boutique ones, or there are B&Bs on there too.....like earlier poster...get a private room and one still would have new people to talk to in the social areas. (Though personally..I won't go for the Chungking Mansions... :-) ) Can see pictures, etc..on the website. Some cities have better hostel infrastructure than others.
Luxury hotels look all the same after awhile...and so do big cities...
Have done 2 RTW (5+7 months) and stayed at enough 5* hotels in my life...and have been to all those cities. Planning excursions and overnights to smaller towns may make things more interesting...unless you (two) really love big cities. Though it may just all be a blur if that is all you do.
Too much of anything....it's all relative to one's frame of ref though.
If you go for a hostel, don't like it...then one will appreciate the 5* place that much more...