The guys are very different. Grant's expertise is in designing boards with lots of volume, which makes him popular with the heavier or older surfers, or for guys who don't surf often, or for guys looking for a small summer conditions board. Grant only shapes with traditional PU blanks he's not a fan of Epoxy, parabolic stringers and all the other new stuff.
Sunova use new ideas, e.g. the parabolic stringer and composite sandwich construction. It's kind of hard to compare these two shapers.
I've only ridden Millers. I have a custom F-Skate which is a good board but a fair bit too much volume for me. I'm still fairly young and don't need that much to get me going. So I find it a bit too corky as I'm used to performance short boards, but for a fish type board it's not bad.
Grant's a very approachable and friendly guy and is real nice to deal with. If you are interested I'd recommend you pay him a visit but S.A. is a bit far from his shop at Mona Vale. If you want more feedback on him check
http://www.waterskatesurfboards.com/testimonials.aspx
That's the Miller side, sorry I don't have any experience with Sunova.