Rocket explained the difference between V2 and V3 sensors very well. As for models line up, you have to see at least 2 different Casio sites to see the full line-up for Protrek. The number of available models varies between region to region.
The Official Casio Clock Site. Find the watch that's right for you from our abundant lineup. G-SHOCK, BABY-G, PRO TREK, EDIFICE, SHEEN, Clock
www.casio-intl.com
The official PRO TREK site. A rich assortment of PRO TREK authentic outdoor gear products. This lineup is optimal for mountain climbing, trekking, and fishing. CLIMBER LINE, ANGLER LINE Smartphone Link, CAMPER LINE
www.casio.com
It's actually very simple. Depending on whether you want Digital or Analog-Digital, you can eliminate half of the line up. As you will see, there are many overlapping models with little or no difference in functions, but only differ in exterior designs. There are also many models that I think are going out of production.
Digital:
There are still older models with the V2 sensors. For example: PRG-240, PRG-250, PRG-260, PRW-2500. PRG has no Radio Control, whereas PRW has Radio Control. All the rest of the functionalities are very similar. The only functional difference: the 240 and 260 have sunrise/sunset time; the 250/2500 have moon phase and tide. 240 is 100 meter WR, 250/2500 and 260 are 200 meter WR. All of these are very old and I think they are phasing out of production, but you can still find new ones depending on your region.
Then there are models with the V3 sensors. For example: PRG-270, PRG-300/330, PRW-3000/3100, PRW-3500/3510, and PRW-30. The 3500/3510 are 200 meter WR, all the rest are 100 meter WR. The 270, 300, 330, 3000 and 3500 have no STN display. With the exception of 330, all the rest in this group are older models and I think are phasing out of production. 3100 and 3510 have STN display, which improves the viewing angle. These two I think are also phasing out of production, and slowly becoming higher price and harder to find for a new one. The PRW-30 is new, it's slightly smaller and has quick release spring bars. It has not been released in US yet, but available in other regions. Regardless of which one you choose, all of them have basically the same functions! None of them has moon / tide, all of them provide triple sensors and sunrise/sunset time. The only difference being exterior design.
Analog-Digital:
Casio was going in favor of analog watches started a few years ago for G-Shock, and the same trend extended to Protrek. Over the last few years we have many analog models, such as: PRW-6000/6100, PRX-8000, PRW-7000, PRW-6600, PRG-600/650, PRW-50/60/70, PRT-B50/70, and PRW-6800. Casio is doing themselves a disservice in coming up with so many overlapping models. Rest assure all of them share very similar functions, as they are all using the V3 sensors. The 6000/6100 was once very popular, but I think are going away. The PRW-7000 has moon age and tide, and I think is also phasing out of production. The PRX-8000 is the most expensive model, but has the exact same functions as the 6000/6100. The 6600, 600, 650, 50, 60 and 6800 are all the same watches with different styles, with the two digits models being smaller. The PRW-70 is a smaller version of PRW-7000. The PRT-B50/70 models are Bluetooth connect models, and the only ones with no solar charging.