My opinion comes more from many years of freeride and downhill mountain biking, but in my experience there is virtually no concern for field of vision when riding off-road. The main field of vision is very large and the peripherial vision is just not a factor in off-road riding: you are concentrating on the trail and obstacles in front of you (it doesn’t matter if you don’t see a bird 170 degrees off to your left).
The only time it should even be a concern is when riding in the city in traffic. In traffic it can be important to not limit the peripherial vision at all, as moving obstacles from the side can be dangerous (cars).
As to specific models, it’s mostly personal preference. The biggest problem riding with a full face helmet is overheating. Also consider that the lower speeds of muni compared to mountain biking make the ventilation systems less effective. And slogging uphill is hot in almost any full face helmet.
That said, I do use my full face helmet for downhill muni (either uphill via lift or I take the helmet off uphill and put it on just for the descent).
The most important factors are fit, comfort and heat dissapation (don’t get black if you ride in the summer sun). Factors like removable liner or washability may be important if you sweat a lot. Weight is generally but not always a factor of what you pay (the $350 carbon helmet will be lighter than you $80 basic). Side note: I have preferred most of my not-so-expensive helmets to the one super expensive one I bought, simple because they fit better.
There aren’t many bad helmets and it’s mostly trial: as each persons head shape is different, one person’s favorite may be horrible for someone else. YOU NEED TO TRY THE HELMET ON, or at least have the oppotunity to send it back if you mail-order it. If you’re ordering online without trying on first, I would plan on ordering 3 helmets and trying them all and sending 2-3 back. A helmet that too big sucks and is unsafe, too small or wrong shape is uncomfortable and you’ll hate it.