Its not really a case of which is a better brand as they are both excellent. However, I would always wear wool. The Icebreaker merino wool tops are awesome. Wool doesn't smell or itch and dries quickly. If you look around you can find merino wool clothing a lot less than the branded Icebreaker stuff. http://www.norwegianstore.com/index.html
This place was run by an Italian bloke and he sold really cheap merino clothing. I got a Dale jumper over 10 years ago and it is one of the best things I have ever bought.
Also check out Ullfrotte clothing. Probably the best you can buy.
I plan to buy a Merino wool base layer soon. My Helly Hansen blistered for some reason so I don't use it anymore. It was good temperature-wise, but it was offensively smelly whenever the weather was a bit warm and/or a bit humid, within a few hours of relatively gentle exertion. Not brilliant for hygiene.
I'll post my impressions of a wool base layer when I get it. I don't think I'll be getting Icebreaker but a slightly cheaper brand, such as Trekmates, in the Â£35-40 range.
They aren't known as smelly Hellys for nothing. But that is the only real downside to a tried and tested piece of kit.
Merino is the dogs, but has a number of drawbacks, not least the cost. You could buy two Helly base layers for the price of one Icebreaker.
I have owned cheaper versions, Craghoppers in particular, and found them to be itchy.
The other big problem with merino is that while it does wick moisture very efficiently, if it does get wet it takes forever to dry.
Helly now do merino/lifa mix so that may be an answer, but I dare say they cost at the merino end of the price range.
I've used Helly hansen stuff for years but have recently changed over to Icebreaker which i have found absolutely outstanding although more expensive. The best thing about Icebreaker in my opinion is they dont stretch or smell after prolonged use.
Trouble with Icebreaker/Merino is when it gets wet as LanceBombardEars said - seems to stay soaked, and also stretches away from the skin, at which point it's just a cold wet flapping mass of sheep hair, instead of something trapping air close to your skin. Got badly caught out on a very wet OMM using these and have sworn off them since for anything where I'm likely to get a soaking.
On the other hand they're great for skiing holidays and the like where social mingling (as opposed to minging) is on the cards.
HH smell can be got rid of with a quick soak in washing soda solution (soda crystals cheap as chips from supermarket)
HH all the way i just find Merino itches maybe that just me(I never could get on with jerseys heavy wool either). As for the famous Helly smell,Ive never noticed it but they say a fox never smell its own hole,Ive been told the later Lifa tops are treated with an anti bacterial agent which prevents the smell. Got to say they have seen me through a lot of very cold days.
To return to my previous post about Trekmates base layers, I've worn both base layer and long johns on a couple of hiking and OTC trips and it's very warm, never, ever smells, comfortable, stays warm when wet with sweat and that sweat dries VERY quickly. I've gone from drenched-in-sweat-after-a-tab to virtually totally dry in 20 minutes simply because of body heat,
on a cold humid day. It seemed to dry from ordinary water quite quickly as well. It's actually disturbing how it doesn't smell at all despite wearing it 24/7, literally, several days on end and sweating like a pig in them. Well worth the Â£65 total cost, as far as my small opinion goes.
Thumbs up for merino. Always worn Icebreaker when it comes to merino wool, but recently found a company called chocolate fish. Little bit cheaper, actually made in New Zealand instead of China for Icebreaker. I find the fit better, little bit longer in the body too. Wore HH in the 80's but things have moved on. They are more expensive but they are more versatile as you can wear a merino base layer in summer to keep you cool.
Swear by icebreaker kit if its cold but dry , like skiing or climbing and general winter use , found it to be very good in normal use MTB ing etc , but as a poster stated if you happen to get caught in very wet weather like the OMM a few years ago then its not ideal , the lifa-wool mix sounds like its worth a go , agree with others about the ming factor of HH's and similar products .
I have seen some merino base layers with an NSN but no idea what the numbers were -doh.
I've always used HH stuff as a baselayer when I'm diving, never felt too cold even when the drysuit has been leaking, and have used it when running over the winter -then again for me the cost issue makes HH the only choice really as I get it through work (aquire might be more apt but don't tell the boss!).