We actually stayed at the Old Faithful Inn in the summer of 2006, and WL last August so it was an interesting comparison. The similarities are striking as you drive up to WL - from a distance they look quite alike but the settings are very different. WL is in a wooded area with a lot of green trees while Old Faithful is open and rolling with the steam from the geyers all around, and the forest is more in the distance at the edge of the geyser basin. Walking inside the Wl you're reminded of Old Faithful but it's obvious that the WL is much newer and bigger (except for the front door of WL). But the common areas of WL are definitely influenced by Old Faithful Inn and we were constantly pointing out details that we'd seen at the older lodge. The guest rooms and dining areas had very little in common, Old Faithful is more craftsman in nature while WL seemed to be more rustic craftsman (emphasis on rustic).
We enjoyed both and it was fun staying in the two of them in a relatively short period of time. If you get a chance I'd advise anyone to at least go in the Old Faithful Inn as it's such a classic and really hard to appreciate unless you're there in person. BTW, there was very little cost difference for the 2 rooms (we got a good deal on WL), and I'm sure for many Old Faithful would make WL seem like a bargain for what you get - very different amenities! I would absolutely stay at Old Faithful again but lodges in the National Parks aren't inexpensive- just like resorts at WDW!