It's very confusing. I have driven both on the street, and I doubt that your "pants seat dyno" will tell the difference. Some of the older GM rect. port heads were about 327cc runner size, and that is considered my most hot rodders as a little big for the street, and lower rpms. I will really flow at 5K and up, but that's where they work best. An average size for oval port intake runners is 260/270cc, quite a difference. Now, the confusing part...you're right, many aftermarket heads and crate engines use rect. ports, and they run strong, even on the street. I have a friend that owns a garage and drag races, and he's a huge rect. port fan...he swears you'll never tell the difference on the street. I dont know if I can make a fair comparison, cause the rect. port engine and oval port that I drove had different cams, pistons, etc....not really the same. But I can tell you that both were animals. If it's strictly a street engine, and 454 or under in cubes, I would probably lean towards the oval ports. If it's bigger inches, go with the rect. ports. From a physics standpoint, the air will move more quickly thru the smaller port, packing more volume in the cylinder. Lots of things will depend on your build. If you're using a stock or near stock stall speed converter, or a standard tranny, the lower rpms will inhibit low end a little, cause you'll spend more time in that rpm range. If you have a bigger stall, maybe 3200 like mine, or more, every time you hit the throttle, you'll pass thru the lower rpms pretty quickly, and the bigger ports will be less noticeable. There are a couple really good books on big block chevy's...look in your bookstore and read up as much as you can on them. It's valuable info, and will help you make the right choice. Do you know yet what you're trying to build?