Nothing wrong with Carrera, I've found them to be really well built, solid bikes, and usually pretty well equipped as well. Both those choices will do you fine and they'll be perfectly nice to ride. If I've got any minor criticism of the Carreras I've worked on, I'd just say the frame welding can be a bit on the heavy / agricultural side (if that's something that remotely bothers you, but at least it makes for a solid, long lasting frame) and they are a little bit on the heavy side compared with say, a Trek, Giant, or a Specialized equivalent... but then there's a huge price difference, so at the 300 quid price point you really can't grumble.
That Planet X bike looks superb value, and it's a bit less ubiquitous, I think I'd be tempted by that if it's in stock. But no, nothing wrong with Carreras.
Are they still made by Merida I wonder?
