Probably not much in terms of performance. Both are a straight through design. I looked at the dynomax muffs when I was looking for something other than the old 2 chamber flowmasters to go with my bassani cat-x. The ultra-flows are the old "race magnums." They're cheaper and louder than the welded. The welded are supposed to retain the same hp gains with a more refined sound (less interior noise)--for a higher price, of course...