First road bike - Ribble Sportive Bianco or Gran Fondo??

I'm a moderately unfit bloke in early 40's looking to become a less unfit bloke and compete in a triathlon in 6 months' time. Clearly I will need a bike for this - sub £1,000 is the budget and the apparently obvious choices are (i) Ribble Sportive Bianco or (ii) Ribble Gran Fondo. Both come with Tiagra gearset and entry level wheels (worth an upgrade from Rodi Airline 4 Clinchers to ITM Aero 2.4 or Shim R501's?). Key thing is the more I read the more confused I get - there appears to be so much choice and so many differing opinions. I haven't ridden a racing bike since my Raleigh Arena days in the 1980's) but equally would expect to 'get the bug' and do regular 20-30k cycles going forward). Would someone like me notice the difference between a carbon and alloy frame, and how would the Ribbles compare with Pinnacle Evaporites from Evans or the Carrera's from Halfords? Other option is of course something like a Specialized Allez Elite or a Cannondale CAAD8 with Tiagra - obviously both of these are alloys but have the advantage of being able to go back to the bike shop in the event of any problems (has anyone had issues with Ribble?)
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The basic wheel upgrade is a no brainer. Not much money for a quite a lot lighter wheelset.