Re:Protein from myprotein.com or Met RX?
To be honest, as long as you are sticking to the brands suggested, ie reflex, myprotein, cnp, bbw, gaspari etc, the 'effect' should be comparable. If it is just a whey product you want, it basically comes down to amount of protein against cost. They are all around 70-80% mark with minimal carbs.
If its a weight gainer you are after, look at the source of carbs being used and avoid ones heavy on malto/dextrose.
After that, it just comes down to which one tastes better to you (its an individual thing) as some are more than happy drinking unflavoured at a snip of the cost.