I've only used mine for personal work, I assume we would use them differently. I would consider these points for myself.
1) Convenience/ Efficiency - When you need your dump truck, would it be ok to take the time to re-configure your Ram to dump trailer work then back? Or would it be more beneficial to save the time and just hop in the dedicated machine? Are you going to be able to maintain and charge the deep cycle dump battery while potentially storing the trailer at a jobsite? They do charge while pulling but sometimes not enough, or the battery may drain while sitting and need toped off. Getting out of the truck to open and close manual trailer gates vs in-cab mounted gate controls.
2) Duty Cycle/ cycle speed - PTO driven dump and Gravity down vs. Battery powered hydraulic pump with a longer cycle time and likely power down also. Can you deal with the extra cycle time and potentially the down time of letting the electric-hydraulic pump cool resulting in interrupted cycles or extra time between cycles? Or do you typically have mostly haul time and limited dumps per day?
3) Maneuverability - I would also put this in the efficiency category. Do your jobsites and dump sites generally have plenty of room to get where you need to be, in the direction that you need to be in? I assume you are a good driver that can pretty well get anywhere you really need to be, but consider the extra time it may take to do the extra maneuvering a much longer rig requires. Just a few dumps a day may not matter, 10 a day would start to add up, 30 a day could potentially be brutal.
4) Loading height - I don't think this would be an issue for you. Is your equipment or any 3rd party loading you deal with capable of loading either one easily and efficiently?
5) Dump capacity - This one is a big one to me. On the dump trailers I have used the trailer payload capacity and dump capacity don't always match. If you put your load farther forward to use the most of your trucks payload then your trailer may not have enough power to dump it. On the equipment I have experience with, pound for pound - the dump truck seems like it has a much stronger lifting system compared to the dump trailer. I can way overfill the dump truck and it will dump, but on the trailer if I don't have the load just right it is on the verge of not being strong enough even if it's under filled. But there may be systems on the larger trailers that are better than the smaller ones.
6) Traction - You probably have more experience than I do with the dump truck vs heavy trailer. I have found that the 2wd dump truck does better than my 4wd truck pulling the dump trailer on softer ground. When moving they are similar, but taking off from a stop is better with the dump truck. Once loaded, in areas that I have to put my pickup in 4wd and ease out, I can just pull off no problem with the dump truck. Also, if I do get stuck I can get the dozer and push the dump truck by myself. I've even done it with the parking brakes on, just lift up the back end a little and push. But you can also pull the front of the truck almost as easy, and I think you put a big wench on the front of yours anyway.
7) Opportunity - Are there any jobs that you do now, or would consider later, that the door would close on if you got rid of the dump truck? Any new potential that the trailer would bring that you don't have now? Can you use the dump truck as a power unit in a pinch to pull trailers if your 5500 goes down, or any other ways that having an additional truck could be beneficial?
It sounds like initial cost would be about the same. You may have a few more dollars in your account, but I would personally consider that even money. Next year you will not remember that $5,000 but you will still be dealing with the results of the decision by either pulling out in your dump truck or hitching up to your dump trailer.
The annual costs would be what I would consider more. Will the savings of your government fees and maintenance justify the lost efficiency of moving to only a trailer? I would expect that you would spend similar money on fuel, tires, and oil changes, etc. either way since the trailer ain't going to drive itself, potentially more since you would have more tires to deal with.
