ultra_man,
Welcome to Tool Talk, and as Papaw pointed out this site is mostly for old tools. That being said, you have asked a valid question. I would recommend the Dremel variable speed unit. It gives you much more flexibility, especially if you are using the wire brushes that are available.
I would not attempt to port cylinder heads with one. I used a Craftsman high speed heavy duty grinder for cylinder head porting and "slick shifting" the transmission sliders. I'd still be working on the first one if I would have had to do it with a Dremel. As with most things, use the right tool for the job, it will make your life a lot easier.
Just my two cents worth.
Mike