I think it depends on the individual and their experiences with these salts. I've tried using only Dead Sea Salts, and compared it with using only regular Sea Salts mixed with some Epson Salts to increase the magnesium content, and I did not feel enough of a difference to justify the additional cost.  But I'm glad I tried Dead Sea Salts so I had a basis for comparing it to regular Sea Salts with Epsom Salts.

Why it does not make sense to use the bathtub for the Sea Salt Bath?

With a bathtub of water, you would need a lot more Sea Salts and Epsom Salts to get the salt level high enough to make a difference.  It's also more difficult to keep the water hot enough in a bathtub without adding more hot water, thereby diluting the level of salts.  Lastly, it more difficult to adjust the temperature of the water if it's unbearably hot.  This takes a bit of experimenting to find that comfort level where it's hot enough to do the job, but not too hot to burn you.

Why it does make sense to use a small wash basin tub?

The tub I use has about a 3 gallons capacity.  I have a glass pot that's just the right size for boiling and reboiling the Sea Salt water.  I use less than one gallon of water because with such a small amount of water, I can easily adjust the salt level, quickly boil and reboil the water, and save money on Sea Salts, because it takes a lot less Sea Salt with a small tub.  It's also easier to use a towel to control the level of heat and stinging from any open wounds. The hot water dissolves the salts the best.  By using a towel,  I could apply small amounts of the salty hot water to skin tolerance.  When the water got cold, I reboiled it and repeated this process for about 20 minutes.  By using a small tub, I could get away with using smaller amounts of Sea Salts, since I always use new salts with each bath.

If you mixed the salt proportions right, you should experience at least half a day without itching.  Afterwards, don't rinse or dry off the salt water on your skin.  Instead, now apply some Borage Lotion, or Vitamin E Lotion, or Oatmeal Lotion,or Glycerin Lotion to keep your skin mosturized.

In the past, when I was itching a lot, this process was what gave me significant relief.  It takes a little bit of experimenting to find the right proportions, but the itch relief was worth it for me.  I found two part Sea Salts and three part Epsom Salts, optimal for me. 

Also, when using hot water (highly recommended since it's best at dissolving both salts), be sure to use only a glass pot to boil and reboil the salt water per session.  DO NOT use metal pots like aluminum, copper, stainless steel, iron, telfon or silverstone coated pots because the minerals from the salts when heated and dissolved may cause harmful metals to leach into the water.  That's why I bypass this potential problem by using only glass pots.

Good Luck and may you have itch free days!