They look very similar. Is there any significant difference between them?

In a nutshell, the 5500 is much bigger in size. Not sure how much louder.

The 5150 is a medium sized mid-size box, whereas the dimensions of the 5500 put it at either at the top of the mid-size category, or maybe even at the bottom of the large category.

Sorry, no pics to back that up, maybe someone else can visualize this for us?

The 5150 and the 5500 are pretty similar in size.

The 5150 is 19 7/8" W, 11 1/16" H, 6 7/8" D
(source: service manual)

The 5500 is 18" W, 11 1/2" H, 4 1/2" D
(source: Ebay listing)

Sounds like the 5150 is bigger than the 5500 then.

If you put them side by side, they look to be about the same. Very different looks though. Both are nice boxes, but will not bowl you over with bass (5500 is better in that department).