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JKSGC's slabs look much better, they grade more consistently, and regardless of what you are told above, their customer service and guarantee are second to none. Do a search of past posts on psa's customer service and I think you will find more complaints than accolades. Same goes for their guarantee.
As for the registry - sgc's has been overhauled and I dont believe its any worse from a user interface perspective than psa's. I will acknowledge that there are not as many users/registrants. I will also note that psa's pop reports are much easier to use - particularly if you are interested in looking up the population of an entire set as opposed to an individual player.
Finally, I agree that there are more psa cards out there than sgc. It doesnt deter me from simply buying psa and crossing them over to sgc. As long as you dont crack the cards out before submitting to sgc, you will be fine - if they dont cross, simply sell and try to find another card to fill the slot.