I tried a heather mead at IMF in 2007 and the brewer back sweetened with some heather honey because it is so expensive in the US. AWESOME flavor and if you have access to heather honey go for it!! Depending on the price another option is to add heather tips in your mead while you are fermenting it. Really nice subtle aroma and flavor from the heather tips.
Both yeast you have would be fine just build your must around the final product you want keeping in mind the alcohol tolerances of the yeasts. Make sure you add proper nutrients for them too.
For folks in the US,
http://www.beefolks.com actually have real Scottish Heather Honey. Expensive, awesome taste, but expensive for a straight heather honey mead.