In general I don't really prefer either - I think depending on your needs sometimes is Deltec and sometimes its H&S. If you need a HOB, only Deltec offers them. If you want an in-sump, only H&S offers them. On many like models, H&S is much less expensive (at least here in the US, not really outside of the US). There are two things on the cylinder skimmers that I do prefer on H&S tho. H&S (and bubble king) use acrylic above the cylinder and Deltec uses ABS plastic. You can't see thru the ABS and thats an important area to see when fine tuning them. Acrylic is more expensive to work with but I think the choice to use it in this area is worth it. H&S (and bk) also use a wedge pipe, where Deltec uses a ball valve on some models. The wedgepipe is super easy to adjust, I found the ball valve to stick a little, and its harder to fine tune them. Sometimes it just comes down to size, in like models sometimes Deltecs are taller and sometimes H&S are taller. They're both incredible brands, for my spare no expense system I got a H&S, but I also just got the MCE600. My favorite skimmer ever tho was the A200 2x1260. It skims things out as fast as you can add them. But in some like models the cost difference is dramatic - you can get a H&S A150 for $650 and the AP600 costs close to that but uses a smaller pump and much less acrylic.
Sorry for the thread hijack LB!!! Both companies are RC sponsors and have some very informative forums!