The biggest difference between the cameras is that XL1 has removable lens and the VX2100 does not. This fact alone makes the XL1 a more "professional" camera than the VX2100. However, the cost of optional lenses places the XL1 in a much different price category, where the competition is steeper.
From a 'real world' usability standpoint, the XL1 is an awkward camera to use hand-held, due to its shape and as you point out, the XL1 does not have an LCD screen. IMO, the XL-1 is suited for tripod shooting in controlled studio conditions. The VX2100 on the other hand is designed specifically for hand-held usability. It sports a stunningly bright LCD. It is well-balanced and its attached handle allows greater flexibility in acquiring different types of shots, espicially the "floor up" variety. The VX2100 is an ideal weight for steady shooting, especially compared to lighter cameras that make it difficult to guage the camera weight when you're trying to hold the camera steady.
The picture quality between the XL1 with its standard lens and the VX2100 is on a par. Sony fans prefer the VX2100, while Canon fans sing the praise of the XL1. The point is there's no clear winner there. You can definitely get broadcast quality footage with the VX2100. Also, it feels more rugged than the XL-1, which somehow seems delicate.
In conclusion, I recently purchased three VX2100's for my business - and so far, I'm totally happy with them. If I was going to be hanging off the side of a mountain shooting a camera, the VX2100 works great for one-hand shooting, while my other hand would be gripping a rope rather firmly. However, it would be unfair not to mention the Canon GL-2, which is of a similar design to the VX-2100, and although IMO it doesn't quite match up to the VX2100, it takes a good picture and is significantly cheaper - in case you have to drop it :)