Is there any difference between the two?
club soda usually has sodium added, while seltzer is usually just CO2 carbinated.
Club soda has sodium bicarbonate which (in my personal opinion) gives the bubbles a harder stinging fizz, Seltzer merely has carbon dioxide dissolved into water under pressure, which bubbles out when you open the bottle.
You can make out a slight difference in taste when you leave a bottle of each open to go flat.
I can taste the sodium in club soda and prefer seltzer. I lived in Toronto for awhile and at the time it was hard to find seltzer, except in speciality stores, and I bought all of the equipment to make my own!!
spot on answers...for a refreshing hot weather drink try a "sonic"...half soda half tonic with a lime! mmmm!
Here's a bit about making DIY mineral waters with the sodastream. I've tried some of the suggestions just a light touch of baking soda, and salt, and a few flakes of Epsom salt.
And it did give a nice fuller flavor like you'd find in bottled mineral waters.
You can, in a pinch, substitute one for the other, but club soda as jwolfsthal said, does have some sodium added