All the HHA sights are top notch. I've used the OL-5519 and DS5519 on my bow and have sold many of the XL series and the OL5000.
Realistically, it's going to come down to personal preference. The OL5000 has no rheostat and 3ft of fiber optic. It's bright, plenty bright for most people in daylight and lower light on a cloudy day.
The 5500 series has 5 ft of fiber optic and the rheostat feature. It's bright enough for lower light situations near dawn and dusk and the rheostat feature allows you to "turn it down" if it's a very, very sunny day and the pin is too overpowering. The rheostat feature also comes in handy with indoor shoots. I turn mine down indoors b/c the flourescent lights and the intense concentration on not messing up using back tension can make my pin a bit fuzzy when it's really bright.
The XL series uses 10 ft of fiber optic...I think you can see the pin from space. It's very bright. The XL series also uses a larger diameter housing (2") as has been noted in the posts above.
In regard to the XL over the regular OL5000/OL5500/DS5500, you need to make sure that you match the peep with the housing. For the larger diameter housing, you need to use a larger peep. This will allow you to align the peep diameter with the diameter of the sight housing when at full draw. For me, it makes it much easier to acquire my target and increase my accuracy when the peep housing and sight housing line up at full draw.
Hope this helps,
Jim