Direct drive is good for a new van with a warranty. Slide in is great for used vans you plan to go through while your TM still has life in it. My old boss has shuffled one slide in through 3 trucks already.
I have both. I bought a brand new Hydramaster 4.8 CDS in 2018. I use it for commercial, residential and water damage. I don't have any 500' runs on any jobs so the heat, vacuum and functionality are perfect for me.
I think honestly a lot of it comes down to the market you're in and what kind of van you purchase. If you have a lot of suburban residential clients then a nice high top van with a big slide in like an Apex 570 just carries so much presentation value (and is more powerful than a DDS).
If you have a Chevy Express or GMC Savannah type van and need to save space I'd go for a DDS.
I think overall the DDS is a little less maintenance but also more wear and tear on the vehicle.