Drop checkers are drop checkers. The fewer number of rubber seals, the better. The cheapo Evilbay single-unit glass dc's will last longest. However, if you're looking for something cheap and plastic, search Amazon for "CO2 drop checker" and you'll find a similar unit for half that price.
How many drops of ph solution are you using with 4kdh solution when using up aqua's drop checker? Instructions say 5 but the solution appears to be really dark. I'm having trouble seeing changes despite increasing CO2.
Aesthetically, I think the ADA one is the best. The cal aqua nano is probably the smallest. The older one was better looking than the new one which I currently own. It's still pretty sweet for the price. I would imagine the any of the white plastic ones are the easiest to read.