FYI - Character slots discounted to 640 gems, but HoT/PoF deluxe upgrades are 600 and include a character slot each.

If you do not own the deluxe upgrades for HoT and/or PoF and were thinking about getting extra character slots anyway, you can get the slot plus other bonuses for 40 gems less than just the discounted character slot alone right now.

The PoF one also has a lounge pass (Lily of the Elon, near Amnoon), which is very nice if you don't already own one. They both have a few cosmetics.

It doesn't show a time in-game when the deal will end unfortunately (instead just shows the "1 available at this price" message) and I couldn't find that info elsewhere.