It is not snobbery to trust one £300 frameset over another £300 frameset, both from places that focus on good value for money. The Spa offering is known to ride well and has tubes that we know about. The Planet X is more of a gamble.
Thanks for all the input thus far, I think the view above is a good summary of the respective merits between a known quantity (Spa in this case) and Planet X, which appears to be so new that there is no apparent track record in a frameset of this type and so little to make an informed decision on. My only objection to the former is that it is black only, but then there is a strong case for function over form, I guess.
Thanks again for your thoughts