I'm not sure I agree that Stephenson is awesome. I've read a few of his books, and I'd rate them 3 out of 5. I don't think Stephenson has the same feel as Williams at all. But I do understand the recommendation as he writes a lot of cyber novels.

As Legolas said above, Williams is epic fantasy at his core. So even though Otherland has a sci fi premise, it reads like epic fantasy.

I'm not sure by reading your post if you are looking for more of a Tad Williams style, or a sci f premise. If sci fi is the goal, I would agree with Tigr below recommending the Ender series. Enders' Game is good, but as it gets into the second book, Speaker for the Dead it becomes excellent.

Williams has two other epic fantasy series that are quite good. I also recommend Kate Elliot's Crown of Stars for similar epic sweep.

I'll think about other rec's and repost if one comes to mind.