Yes, Space Mountain is better, you're right, though I kind of wish we had a soundtrack and the option to plop in Hyperspace every now and then. WDW's Haunted Mansion is the one I grew up with, so I'm biased towards it, but I kind of feel like they're pretty similar in terms of experience these days. WDW's queue is better with the outdoor enhancements from about 10 yrs ago. But DL does the Holiday overlay, which is really cool.
When I wrote to Don Carsen, one of the lead Imagineers on WDW's Splash (see several hundred pages ago on this thread for the transcript), he told me that the attraction has really good bones and would hold up well with whatever changes they needed to make to it. That gives me hope that perhaps the WDW version will still stand out in some ways. What those are, I don't know, besides maybe the fact that the logs are more comfortable and the overall speed is more relaxed. We'll see.