I have a Planet X Sportive with an Enigma Ti fork, both quality products and I think anything you get from either company would be great. But before you go spending you money, you might refine what you're looking for. The Eclipse is far more of a road bike than the PX Sportive or Linskey Cooper. A better comparison from the Enigma range would be the Etape. The Eclipse is also plain gauge tubing rather than the butted of the PX and other Enigma models (I don't know about the Cooper). I've no idea how much difference that makes, but it goes a fair way to explain the different prices.
If you can bide your time Planet-X have frequent short lived offers, My Sportive came with a free carbon frame and fork in a one day offer! The present offer of a complete bike with Dura Ace for £1999 looks good value when you consider that groupset costs £1000 even from the big discounters.
Jim Walker has a reputation of being very friendly and good at offering advice, I also found Planet X helpful by email, though they don't offer the same level of bike shop experience.
Also consider the finish, one of the advantages of Ti is the natural finish, a scratch or mark on a brushed or polished frame can easily be brushed or polished out. That's not the case with a bead blasted frame like the Cooper or some Planet X frames.
The two bikes on my short list were the Enigma Etape and the PX Sportive, I’m very happy with the PX, I expect I’d have been just as happy with the Etape.
Have fun choosing.