I'll also throw in a +1 for the ph100/ph110, although I have no experience with the pH Tester30. Regarding Extech, the difference between the ph100 and ph110 is that the ph110 ships with a refillable probe module, the module on the ph100 is not refillable(as you noted). You can interchange tips though (IE buy the ph100 and then replace the tip with a tip for the ph110). I bought the ph110 myself- I will note that, ironically, I went to refill the probe recently and broke the reference junction following the instructions in the manual so I am feeling a little irate toward the ph110's probe.
Again though, no experience with the Oakton, hopefully someone here has some perspective on it (I would be interested myself). I do have a friend who uses Oakton pH meters and thus may have used it- I will ask next time I see him. Personally, given the bulb style tip, I would be hesitant. I went with Extech because of the flat tip.