No comparison.

Thuja occidentalis 'Green Giant' all the way.

Stay away from Emerald Green AKA Thuja occidentalis 'Smaragd'.

Make sure they a single leader specimens.

Start with 2/3 footers. Add no amendments to the soil.

They grow quickly.

Ken will probably chime in with his 2 cents. Pay attention.

Dave

I pondered the same question a while back, after buying a house with a landscape plan that called for Emerald Greens and learning that the previous owner planted Green Giants instead. The difference is night and day, as Dave pointed out. Green Giants get a lot taller and wider than Emerald Greens. You'll need plenty of room vertically and horizontally if you want the former. I ended up having my Green Giants moved to a friend's house because I felt they'd outgrow the spot they were in. My friend was happy to get them. Personally, I prefer the foliage of the Emerald Green.

Green Giants all the way here... plus they aren't as over used as the Emerald's that you see in every big box store in America. Get them smaller in a one gallon container... usually they are around 2' tall. They grow fast and in a couple of years start to produce a little privacy. Emeralds would take a very long time to grow to that height.

Just to be clear - these two plants are not all similar and only related by genus.

Thuja 'Green Giant' is a hybrid of T. standishii (Japanese thuja) and T. plicata (western red cedar). More of a Christmas tree shape and less narrowly columnar than Thuja occidentalis 'Smaragd' (Emerald Green). And grows a whole lot faster and taller!!

I personally think the GG's make a much more natural looking screen - the arbs are typically too soldierly looking but very serviceable where space is a premium.