I bought a second hand casio pro trek triple sensor that I have now used for about eight years. I have found it to be accurate, reliable and robust. The height readings can be thrown off by sudden drops in pressure, but these can be quickly corrected with a referance to the map and an on the spot adjustment.
I regularly check the compass and it is always spot on. I also find height alarms are useful.
I know you were looking for a review of the Stormshield and it looks like a good buy, especially the metallic ones.
It is very difficult to predict the weather at the top of Nevis. In September you could bake, freeze, get lost in a white out or think you will never see so much rain ever again. May be even all of the mentioned in one day.If weather permits on the day go via the CMD. Best to go mid week if you can to avoid the crowds. (On most weekends there are organised events.) If you end up doing the whole walk on the tourist path promise yourself you'll go back and do it justice next time.