Since you're staying with a van, already know about your current setup, and are very familiar with it, you can keep all your spare parts simplified with the same machine. You can troubleshoot faster when problems arise, and can train your future technician faster, because they just need to know the basics of operating the machine and more importantly, how to properly treat your customer and give a good service. So keep your operation as simple as possible. Stay with the same machine. Don't get a slide in.
Hydramaster's PTO has the best design for more storage space for a van. Which is major. Slide ins take allot of room with the "T" design of the machine/recovery tank.
If your original machine was a slide in, I'd say to keep the same slide in brand/model if possible, unless it was a tiny slide in. Then I'd say get a bigger machine of the same brand/design if possible. Keep your business as simple as possible.