I've used mostly the 442(700) for long time. That doesn't make it any better to you because I do though.
What ever one you get, understand what that conversion factor is, so when you post about ppm #'s on forums, you can also tell them at what conversion factor that ppm# is based off of
Or look at a chart and convert it to EC, so when you post online you can use the universal EC instead
Or get a meter that does EC, or, some are switchable between .5, .7, and EC.
I used 500 (TDS) for the longest time. But ec is universal. 700 just hurts my brain. Therez actually another one i believe...650 or some shit. Confusing to say the least. EC and 500 (which is half of ec without a decimal) works for me.