I have yet to use a device from Manley that I did not absolutely love. I have not had a chance to audition the new Core, but if I had to guess, I'd have to guess that it is filled with that same mojo.
I have also yet to find anything from UA, including their plugins, that I do not absolutely love. I have used the 610 preamplifier and it is very cool.
So, as a guess, I'd have to guess that you can't go wrong here, and the choice will come down to your personal preferences, and your application, and maybe your bank balance? Most dealers will let you trial UA gear, and I think you should be able to trial the Manley as well - that's your best bet... unless you end up wanting them both<G>!