First, they are both excellent bikes so no matter what you chose you will be well served. Second, don't get sucked into the crap above arguing about carbon vs aluminum, it has nothing to do with your question.

The biggest difference between the bikes is the style, one is an endurance fit and the other is more of racier fit. The Ribble will have you sitting more upright with a longer head tube which can be a more comfortable bike for some. For others they want a bigger saddle to bar drop and more aggressive position for that racier position. One isn't inherently better than the other, just different. You have to decide what kind of fit you want on the bike.

Component wise they are equal with both having 105 which is an excellent choice. Frame material there is a difference between the two different types of aluminum but for the average person they would not be able to tell the difference between the 6061 or 7005. Each frame has its merits. The next biggest difference are the wheels. I am not well versed in aluminum rims to tell you which is better value. They are both made by high quality companies so I wouldn't be too concerned.

On-line there seems to be limited stock so this may be your limiting factor, being able to actually purchase one. Last colour matters for some, at least for me and we all know red bikes are faster. Good luck with whatever you chose.