They come from different plant sources, so from that respect they would not be equal.
From the fermentable sugar content respect, again I don't believe they would be equal. I was just reading on dextrose a day or two ago, and in my readings I found out that you would need more dextrose than sugar (12.5% more to be exact).
Hope this helps!
Edit: In my opinion, I would substitute dextrose for white sugar in a cider any day. Dextrose (glucose) is generally a grape derived sugar and would certainly lend itself well to a wine-like end product.