I'm not an expert but know about the two I have:
cheapo $30 one (don't know the name off hand but it's yellow with a black cap) read out: X.X
similar to this one
A bench top one made by (or for) Corning It has a hand wand on a ~two foot wire, it would have likely cost a few hundred dollars (long story, but I payed five bucks) read out: X.XX
looks like this one
Both need to be calibrated with a solution. The cheap one can only be calibrated to one point (I use 7). It seems to be right on at 4 after this calibration though.
The fancy one needs to be calibrated to 4, 7, and 10. This is a PITA because it is a rather complicated device with too many functions. Once it is set up, it gives a accurate read out much faster, that is to say; the numbers stabilize quicker. I almost never use this one, due to the complication of using it, and fear I will break it.
I'm quite happy with my cheap one, and if I drop it in a mash, i won't be that upset. So I would recommend finding a cheap one (maybe used) on ebay. I think that most of them are listed in home and garden for use in hydroponic plant growing. If you really feel you need to see .01 resolution there are some $100 hand held ones that claim to be water proof