The quality of both models is excellent. I have the DragonWings 1:400 version as that is the scale I collect and I LOVE it! (Only a CLOSE second to my DW744 Qantas -Wunala Dreaming! ;D )
The two companies offerings are different in a number of ways... I have seen the StarJets 1:500 version and it looks more detailed in the hatches/vents department, (additional small vent indications and anti-collision markers) however, unless you are a stickler for the ALL METAL thing, I also think that the plastic wings/stabilizers on the DW (DragonWings) are more in scale and detailed. I also don't like the assembly joints on the SJ (StarJets) model.
These also differ in that the DW model has plastic gear which can be removed and replaced with "gear-up" hatches, which are provided and put on the (also included) stand. The SJ model has fixed non-removable metal gear and no stand. How you display it can make a difference... The DW models have more radical "Inflight" dihedral angles on the wings. The SJ's have purposely "non-flying" at-rest dihedral to them. The SJ also seems to have generally more accurate paint than my DW model, which is ever so slightly off square in the nose-on view, but not noticable from any other view.
The difference between the DW "SPL" and "SPK" models is that the SPK is a re-release of a corporate run Dragon did for Singapore Airlines after they saw the public release of the SPL version. The SPL version has some minor flaws in the scheme from a top view, as they didn't have a photo of that angle when making the model. This has been corrected in the SPK version... that, the logo on the stand and the reg-no. are the only differences between them.
Is the SPK worth more than the SPL version? Who knows? It all depends on your pocketbook vs. your desire! The SJ will be cheaper on the whole than the DW due to size, release date and volume of production. But if you count how many "Tropicals" of all variants DW has done, then I think the SJ will eventually be a shorter run and its price will still be less right now as it's just been released.
So... you have to "pick your poison" as they say...