840 pro esp the 256gb version is an absoloute beast,the cheaper non pro 840's use cheaper tlc nand with a clever yet unproven controller to extract fast speeds from it,the pro uses mlc nand which is better
Thanks for the info everyone.
It was the 250Gb 840 basic on today only I had my eye on but I can get the 256Gb 830 for a couple of quid less-but I read that it was a fantastic performer and some review comments stated that it was still better than the 840.
AnandTech had 2 840 Pro's for testing and both died, so a bit early to say at the moment that it is going to be the best drive to own.
The 840 Basic though with performance similar to the 830 looks like a real good performer.