You may appreciate the flexibility of a monorail if you can deal with it being a little more annoying to lug around with you. My Sinar F1 (monorail) is pretty compact and fits nicely into a medium sized backpack. That said, if you'll be doing a lot of backpacking, I'd lean towards the field camera. Monorails are generally cheaper, as well. You could get a decent monorail and something like a Speed Graphic for the price of most moderate field cameras.

I've seen quite decent monorail kits selling for $300-400. Look around for someone who is selling it as a kit so you can get the whole shebang and not need to spend money on the dark cloth, lupe, lens, etc separately. If you've used the Calumet Cadet like I did, just about any half-decent monorail you find will blow you away (If you hadn't noticed, I'm not particularly fond of the Cadet).

In my experience, you'd be hard pressed to find a good, proper field camera for less than the $600-700 range.