But is there actually any difference between them?
Yes, about 10fps at ultra settings but it gets you near or past 60fps barrier most of the time.If it is $15 extra, consider it.
You will want that 3GB VRAM for future games.
Here is a review of a HIS R9 280: http://www.guru3d.com/articles_pages/his_radeon_r9_280_...
I would choose the 3GB R9 280 over a R9 270X 4GB.
The R9 280 would still be faster than a R9 270X. Overclock it further and you can reach near stock R9 280X