The later, the better. Date code up to 1A (March 2011) is good if you want permanent CFW, which is much preferred as it gives you more control and doesn't require running HEN every boot.
There are slight hardware differences between each model revision. Every few revisions, CPU/GPU dies shrink in size for better efficiency and temperatures. Older fat models have faulty GPUs, all models up to early slims have capacitors that don't last very long, and later slims have disc drives that are separate from the main board (but more tedious to put back together). Late slims also have thermal paste below IHS replaced with a solid compound that's supposed to last longer (which appears to be the case), but makes it virtually impossible to separate the IHS for maintenance.
Of course, in any case, I wouldn't exactly recommend the super slim because it's very cheaply built, however from hardware perspective, it is far more efficient to my knowledge.