Beamish has a slightly sweeter taste and fuller, more complex flavour and I think this makes it nicer. I am by no means 'into' my beers but the two drinks really do taste totally different (I'm addressing that at people who say they don't). Guinness has a much more bitter aftertaste which isn't great.
I think the fact that Beamish is sold cheaper, people consider it to be a 'cheap' imitation of Guinness... which it isn't. The drink should really be marketed properly and sold on its own merits, I think lots of Guinness drinkers would convert if they gave it a chance on this basis.
On a side note, a mate of mine's da was a master brewer at Guinness for years (now retired) and he is a Beamish drinker. Says Guinness is gone to ****e these last few years, apparently.
I am going to the offie to buy 2 cans of beamish now!