Good luck! It's not easy finding a truly opaque white pen. I do like the new Sharpie Paint Pen with the fine point though (it's a water-based paint). A bit off the subject, but if you ever look for a nice gold or silver pen to do edging, etc., check out the ones made by Krylon. They're really great. I don't believe they make a white, however.
For pens, I use the Uniball Signo, BUT order then direct form Japan. They are awesome.
I have heard good things about the Dylusions, next thing on my list is all the new colors!
Is there a trick to the Signo pens? I have owned 3 and they work the first few times I use them and then stop. I have tried EVERYTHING with them and have resigned myself to never buying another.
I currently use my white Sakura souffle-- I love it but I really liked the white of the Signo-- I just won't spend that kind of money on a pen to use it twice and have it quit.