#1 Location. Where are you going does that hotel chain have a property in that city you are in? Quantity speaks a lot. Sheraton is huge in Africa and Europe. Total hotels in 6 chains 700 Hotels, Marriott 7 chains/brands 2,000 hotels and the largest is Hilton with over 2000 hotels in 6 brands.
#2 What do need or want upgrades,suites, free breakfast concierge floor etc.
#3 Redemption levels, where will you be using your free nights and how many points does your chain require. Study the program books and search which one you thinks gives maximum value. Beleive me point accumulation is a game, play it right and hello free nights. Some chains also offer off peak for lower levels a great deal at times.