The proposed SDXC spec was just announced
during CES reaching eventually 2TB and 300MB/s.
Will this be an alternative to SSD,
and why wouldn't I want SDXC instead of a 1.5TB hard drive, aside from the initial high cost?
Down the road, will we have our entire archives in our pockets?
Blame the press for confusing you.
SD, SDHC and SDXC all use flash memory, and are "solid state devices" just as much as what is normally labeled "SSD". There are some differences in implementation, due to their different intended uses.
Don't be blinded by the promised future
capacity of SDXC. The first SDXC cards will not
have capacities competing with any large harddisks.
As for whether you will be able to fit your entire archives in your own pockets, it depends on what your archives consist of.
Today, you can get reasonably priced 64 GB USB memory to stuff in your pockets, and capacity roughly doubles each 12-18 months.
An estimate I saw recently (I think it was on The Register) in growth rates for solid state disks and harddisks suggested that they will reach capacity parity around 2021, at 32 TB or so. If capacities grow to be that great, you will probably have easily pocketable 4 TB media.
If you're not even more confused now, I'll smile.