Both forms of multi-board solutions have their perks and weaknesses. They both take different approaches to dual-board configuration.
NVidia's SLI is still based on optimizations for specific games, so unless a particular game has been optimized for SLI (in the drivers by NVIDIA), it will use a default method that doesn't offer good bang for the buck. In SLI's favor though- it doesn't have much resolution or frequency limitations. NVidia continues to add more and more optimized profiles and performance continues to increase.
ATI's CrossFire has two added perks- a variety of SLI methods that are user selectable for ALL games, so the user can pick one of three methods of tiling/board workload. ATI also has SuperAA, which provides superior dual-board anti-aliasing as the sample positions are jittered by each board for better AA sample patterns. ATI's Crossfire does have limitations with resolutions and frequency as the compositing engine between the cards is lower frequency. ATI's Crossfire also (currently) saturates the PCI-E bus with AA as work is done across PCI-E versus the board link cable/compositing engine. ATI promises to change this in a future driver revision. ATI's Crossfire is also limited to ATI Xpress 200 chipset mainboards, which are still very, very difficult to obtain.
So one really cannot say one is better than the other.
In a nutshell- if you are on an LCD and lean more towards cleaner, alias free image quality with exceptional performance, as well as don't mind forking out for a special ATI mainboard, Crossfire will fit the bill nicely.
If you prefer to use an existing NForce/SLI chipset mainboard, care more about higher resolutions, have a CRT with higher refresh capability and want blinding performance, SLI fits the bill perfectly.
Different approaches for different end results.