Just one more thing. I don't know how you use your HTPC typically, but mine is primarily for watching/recording cable TV. So 99% of the time I simply use a regular old IR universal remote. Mostly when I'm web surfing or doing some sort of maintenance do I have to resort to a keyboard/mouse app. And I've used VNC maybe twice in 2 years just to play with it.
Although smart phone apps are cool, fun, and nice to look at, IMO they pale in comparison to a normal IR remote that doesn't require you to look down to use it, charge it every day, wake it up, get interrupted with phone calls, or worry about droppping it. And an IR remote can control volume, run macros and control the rest of your home theater. (Technically a smart phone can too, but that gets expensive and complicated very fast). So I think getting a good IR remote should be your top priority if you primarily use Windows Media Center as I do. If, on the other hand, you think using a smart phone or tablet as a remote is the best thing since sliced bread, then forge ahead with that nexus. I think the nexus does have IR, so that will save you some money if you turn it into a full blown remote for your entire home theater.