If you want it to ride good-ish plan on spending $800-$1*** for some good shocks. I put Elka Stage 1 on my P5, much improved but like @Smitty335 said, there's only so much travel.
I have an 18 500 and access to a 21 520. Both have upgraded suspension (500 has WE - 520 has Stage3 Elkas) and both have power steering. I feel the 520 has much more "snap" from zero MPH. It feels much more torquey. That said - the 500 has 1200 miles on it VS 30 on the 520. Once out of 1st gear - no difference is felt. The 520 feels like it has more throttle response.
I had the same quandary. Chose the dump bed and won't look back. It is the singular most used tool we have now and if I didn't have the bed, I would have spent more than the cost of entry to buy or fab one up, let alone my time. Both seem to be great machines, just different cargo hauling abilities.
After plowing with the 520 last night for an extended period of time. one word comesto mind when comparing engines: SMOOTHER. The 520 motor is much smoother that the 500 motor.