For pickup sprayers I like a variation of the type that's mounted on your pickup. We have a Simpson fifth wheel pull behind sprayer with a 500 gallon poly tank. I believe it originally came at a width of 57.5' and we added extensions to make it 62.5'.

Someone mentioned high compaction with a pickup sprayer. I would think this would be particularly bad with something mounted on your pickup. I would also think that it would be really easy to get stuck with a pickup sprayer that had 500 gallons or so weighing it down. We routinely plowed right through terrace bottoms with water standing in them when we felt like we were getting behind with this setup.

I'm unaware of how mounted pickup sprayers keep from hitting the booms on the ground but the discs on the ends of the booms do a good job of managing that for you with the Simpson sprayer. We've had a self propelled unit (Patriot 150) for five years now and we still pull the old Simpson out to help in a pinch.

I see some of their pull behind sprayers here and I don't see any fifth wheel versions. Both us and our neighbors started out with a fifth wheel version from them though. Our neighbor had his built with a 750 gallon tank maybe 10 years ago and it worked well with him using a Dodge with a Cummins. We used a late 80's Chevy with a 350 in it so we probably couldn't have pulled more than 500 gallons on hills when it was soft. If you give them a call they will probably be able to build something similar.

As far as advantages and disadvantages are concerned, most of them have been covered but one thing I really like about the pickup pull behind sprayer is simplicity. You've basically got a trailer with a tank and a five horse Honda motor. If the motor on your pickup goes out you hook another pickup up in a pinch. If the Honda motor or pump goes out your can replace either in about an hour. It can go as fast as a self propelled unit without the complexity.

 We ended up doing quite a few modifications to our pickup over the years to make it more comfortable while going fast. We hacked the leaf springs (advantage of using an older pickup) with a torch to make the ride softer. We put air bags under it to make it softer and lift it some. We took the seat out and cut a hole in the floor where the driver sits so that we could mount a John Deere tractor seat in it. Our neighbor however sprayed significantly more than we did and never did any modifications and he used a newer 3/4 ton or 1 ton Dodge while we were using a 1/2 ton Chevy.