I believe the Ribble is a bit more relaxed, a sportive bike like a Specialized Roubaix. This is the key difference.
I would go for the Planet X myself but the Ribble is also a nice bike and may be more appropriate if your brother is less flexible. I like a racier geometry myself and have no problem doing exceedingly long distances on one (up to 325km on a one-day last year on my summer racer and substantially more on multi-day events.) Note the cheap PX wheels are out of stock so you would need to spring for the Ultegra 6700 @£300 extra. Still, nice wheels but pricing not the MOST
competitive (remember you need to take the price of the free wheels out too. Maybe email them to find out when the "free" included wheels will come back into stock as they are very decent wheels for the money (both Model B and C.)
I have direct experience of buying bikes from PX and in particular their customer support is very good (I have many emails with them and have bought two bikes.)
I have also bought parts from Ribble and found them fine but there are a fair number of bad reports on them from people buying entire bikes; while there are virtually NONE on Planet X but plenty praising them. Worth bearing in mind.