Shortride,
Essentially SCI is a larger version of DSC. There are some outfitters at SCI who won't be at DSC. But there are also outfitters at DSC that won't be at SCI. But overall you will see a significant amount more at SCI with many attending both shows.
The first and only time I was at SCI, I almost felt overwhelmed it is just so huge. And I believe it has grown since 2011 when I was there. If Las Vegas is somewhere you've been or think you'd like to go, there is certainly all of the entertainment of the strip available to you. All of this does come at a cost as membership in SCI is mandatory in order to attend the convention. This will with tickets run you about $250 or thereabouts.
As mentioned DSC is smaller. This doesn't mean it's small, it's just not quite so large. If time is an issue, I'd say you could cover DSC in two solid days. When I went to SCI, I was there only for two days and wish'd I had had one more day. DSC will cost you less at the door with a 4 day pass running $50 and no membership req't. Passes for lesser days can also be bought and of course are cheaper.
Outside of the costs, the size and obviously location, the biggest difference to me would be the "feel" of the two events. DSC being in Texas, it has a Texas feel to it - easy going, hospitable, not quite so commercialized. SCI just doesn't quite have this quality. It has a bit more of a black tie atmosphere. It's not that I think you'd dislike SCI, I think it's a great show too. But in the end the overall feel of SCI being more exclusive is present. That said, I will be going to SCI as well as DSC this year and I'm looking forward to both.
If you do decide on DSC, please give consideration to joining in on the fun of our 2nd annual AH banquet!