I bought the big one, < 7USD, at a corner, waiting for a green traffic light.
People there, sell incredibly funny, stupid, cheap junk like that which is bought by people (idiots?) like me. In fact, never did before except with (very good) metric tapes, which I use (and destroy) quite often in my job as a Marine Surveyor.
From a practical point of view I am more concerned with the size because your fingers can push two keys at a time or simply the wrong one.
I use, when programming micros, a scientific CASIO. Quite too often I have to clear the entry because of that.
For long numbers in hexadecimal (I just finished testing a binary division routine 32 16 for PICs) found quite useful the calculator coming with Windows but being stuck to the screen is not mi idea of comfortable.