Unless you're already invested a bunch in lenses, and since you've said that $2,000 isn't totally out of reach, I would go with the a7s. That's just one person's opinion, but I don't think it will be long before DSLRs go the way of the HD camcorder in favor of 4k-shooting cameras with incredible (and I mean kookoo-bananas awesome) low-light capabilities.
If (and when) that happens, I foresee a used t[x]i going for a song on ebay.