The express is slef contained 16 channels and has light curves so that the dimming of different types of strings can be matched for a more consistent look.
The Freestyle requires SSR4 or other types of slave modules to do the actual load switching and is locked in on a single linear ramp for the dimming.
A lot depends on what you want to do... An SSR4 is convienient if you have a small tree with 4 colored strings of lights (i.e. RGBW). You can for go the plugs and wire the strings directly into the SSR4 and permanenetly attach it to the small tree making setup and teardown easy.
Even though the smart strings are still an immature technology, I would seriously consider looking at that route. By the time you do 4 strings + 4 channels of control, you are pretty close to the cost of a single string and SSC.
The SSR4 is rated for 1A per channel, 2A with a heatsink. The Express is more like 4A per channel, but even with split power, you are limited to 20A per half. If you want a ton of lights with a smaller number of channels, the express is always a very good choice.
OK, it looks like all my thoughts are random...
One other thought on the freestyle, there are some DC controllers that can be controlled by the freeestyle, just none with the Lynx branding. We tend to steer folks to the MR16 for DC loads as it has a much better control system...