Why not use extract instead of sugar? That would be the best way to go, I think.
But, if you want to use sugar, I'd go with some form of cane sugar that still has flavor to it (turbinado, demerara, sucanat, whatever). You might want to cut back a bit on the bittering charge if you go this route. How much depends upon your efficiency, but a good starting point is subbing 5lbs malt with 3lbs sugar.