I own both. I like both for different reasons.
Wyndham is very flexible, has a lot of locations 75+ all over the country, and is very cheap to buy. However the product is uneven, some resorts are really great and others are not as much, but if you know what the resort offers your expectation will be met. Presidential units are really a wow. Resales have the same rules as everyone else except VIP accounts but it is not worth the price anyway.
Hilton is much higher end and more in line with Presidentials at Wyndham. Prices are much higher than Wyndham, but can be a bit cheaper if you stick to an affiliate. Resort are really great, but are too focused in the same areas, Orlando, vegas and Hawaii (Oahu and Big island). They are getting every so slightly better by adding other locations (Breckenridge, CA, Myrtle Beach), but it is slow. Good customer service, resales no different than direct buy, as far as how you are treated. No Elite through resales, but there are even less perks with Hilton than with Wyndham for elites.
So what kind of experiences and locations do you want to travel, that should dictate your purchase.