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.
yeah, my brother rang planet X earlier, they wont have the standard wheels for another 2 weeks, but then the sale is over, he`s prepared to wait, but i said id research the ribble for him in the meantime
when you say the ribble is less racey, what exactly do you mean?
Specialized probably started the idea with the Roubaix (the Tarmac being the "racier" bike) and it makes a lot of sense for a lot of people, probably most people... The default geometry on the Trek Madone now for example is relaxed, unless you specifically go for the standard crankset "Pro" version or whatever they call it.
problem with the PX model B and C wheels is excessive bite into the freehub from the cassette, well that has been my own experience anyway, and I've heard of others with same experience, other than that PX is a wonderful bike
The PX wheels I got were actually 10 speed only (raised splines) and hence wouldn't even have suffered from that problem. They told me though that most of the wheels were 8/9/10 but if you were concerned you could maybe ask specifically for 10 speed specific.