there is a 150 watt solar panel, 18 volt, 8.33 amps. A second panel is 160 watt, 30.2 volts and 5.30 amps. They are both mono crystalline solar panels. My question is, why so much difference in the voltages and amps? and which would you recommend and why?
To answer your question, the 150w panel is wired / configured as a 12v panel (yes I know it has a max voltage of 18v but it's still considered to be part of the 12v panel class), while the other panel is wired / configured as a 24v panel (yes again I know the voltage is 30.2v max, but it's still considered to be part of the 24v panel class).
With panels it's horses for courses (so to speak).
There's advantages and disadvantages for both (depending on how you'll be configuring your setup), so I'll outline these below to try and help better explain for you.
1. If you are going to be using a 12v battery storage setup (eg 1 x 12v battery or even 2 x 12v batteries running in parallel) and you are going to be using a cheaper style PWM controller, then you really are stuck with using a 12v style panel (as this is then closely matched to your 12v battery voltage). If you are using a 24v battery setup (eg 2 x 12v batteries wired in series) then you CANNOT use this 12v panel as it wouldn't have enough voltage output to charge your 24v battery setup.
2. If you are going to use a 24v panel with a 12v battery setup then you CANNOT use a PWM controller (as the PWM contoller cannot convert the voltage down like a MPPT controller can). So with the 24v panel you would need to either use a 24v battery strorage setup or else use a MPPT charge controller which takes the 24v and outputs 12v (at double the amperage).
Typically as a general rule an MPPT charge controller gives about 20-30% more performance output (from the same panel) compared with the PWM controller, but they do cost more at time of purchase, but personally I'd never usually recommend people go with the PWM controller option as then you are just throwing away precious output each day from your panel setup.
Lastly you need to think about whether you will only ever be running 1 single panel on your roof top, or whether you will want to eventually have 2 or 3 (or more panels) and whether you actually have space for these on your roof top, or whether other items on the roof such as vents / satellite dishes etc are in the way.
Typically if people are only going to be going for 1 panel on their roof (and never plan on adding more) then I'd usually recommend going as higher wattage as possible from the 1 single panel, so either a 265w 60cell (1.65m x 1m size panel) or a 315-330w 72 cell (2m x 1m size panel).
The last thing your want (in my opinion) is to have filled your roof space up with 1 or 2 low wattage panels, then then have no more roof space left available to generate any more power. If everyone had an RV the size of a school bus then sure I'd have no problems installing a couple of low wattage panels and simply adding on more as you need them, but the reality is that roof space on most people's RV roof tops is precious and hard to come by, so I tend to like to generate as much power as realistically possible from any panel that is installed on the roof top (knowing that I don't have space to install as many as I like for more output).
Also just so you know, as a general rule if you are connecting 2 panels together (in series) it's recommended that they be the same wattage (or as close as possible specs wise, eg voltage / amperage output) so that they work nicely together without one making the other panel loose some of it's output.
I hope that best explains things for you, and WHY you'd want to choose a 12v panel or 24v panel (depending on the setup you end up going for).
PS: What size (physically) were both the 2 panels you were looking at, and how much roof space (length and width) do you have available on your roof top for 1 or more panels ? Also are you planning on using your RV much during winter months, as that's the time when ALL Solar Panels will give their lowest output.
Also, the Mono vs Poly really doesn't make much difference to be honest. The OLD style poly cell panels used to have flecks all through each cell (and look almost like the cells was cracked or smashed) but nowdays the new technology poly cells will usually have the same style AR treatments on the cells and so they usually look a nice uniform / consistent deep blue colour, and aside from poly cells being square and mono cells having their corners cut off (typically) there's almost no real performance difference between them any longer (as long as the panels have the same wattage output). Eg I could get a 265w [edit: brand]
Solar Poly and a 265w [edit: brand
] Solar Mono and put them side by side and get about the same output per panel YEAR ROUND. This is a good test as both panels are made by the same manufacturer and are the same wattage. I've personally done this test to see these results for myself here (so I can honestly comment on this as being accurate rather than hype or sales jargon).