With the Leyland Cypress you will have to work harder to control it.

Where's Resin with his picture?

Leylands can be kind of prone to pests.

Arborvitae

Cons
Some leaf diseases
Bagworms can destroy
Deer can destroy
Spider mites can destroy
Heavy SNOW can destroy or disfigure
Slow Growing
Fairly expensive due to high demand and slow growth

Pros
High quality foliage
Wonderful aroma- especially when it's being eaten by bagworms
Easier to control hedge due to slow growth
Transplanting usually successful
Very desirable and makes a good accent.

Leyland Cypress

Cons
too rapid growing for most residential hedges
Can outgrow space
High maintenance hedge can look unkempt not like picture
insects can destroy
disease- canker big problem- expensive removal
I personally don't think they transplant as easily as arb. and or are finicky about soil. I see more dead transplants than with arbs.

Pros
Rapid growthrate a plus for fast privacy
Cheap- $11 at walmart
High quality foliage- dark green tough to beat
Nice aroma
Would make a nice specimen if it wasn't so overused

Just a little suggestion. Why don't you use shrubs as a formal hedge? There are shrubs that grow rapidly but not so tall as to get out of control. You can mix in some trees like leyland cypress or arborviate for additional privacy and for looks. That way it's more like landscaping and less like a need for a screen. If you plant a tall growing hedge in a residential property, while it gives you lots of privacy, it also looks strange. And it makes it obvious that you are trying to screen the view of something.

Some alternatives (depending on climate and site- do your homework)

Nelly R Stevens Holly
American Holly
Japanese Cedar (commonly called Cryptomeria or crypto for short)
Chinese or Japanese Hollies or hybrids (Such as 'Sea Green' Juniper- tall & wide growing dark color. Fast & inexpensive)
Redcedar, Rocky Mtn Juniper, Chinese or Japanese Junipers or hybrids
Cedar of LEbanan, atlas cedar or deodara
Evergreen magnolias- southern, sweetbay
Eurpean hornbeam, American Beech while deciduous offer some dormant season screening.
Chinese evergreen oak
Western arborvitae (aka Green Giant)

Leylands are beautiful but don't root as fast as top growth, so they can topple over in snow / wind / just because.

Just read a site that said they might actually be the biggest tree on the planet if left to grow on their own for a few hundred years in good conditions!

The variation screen is a good idea. I have a lot of varieties of leylands AND lots of other trees, in fact most of those on above list. Variety is really nice.

Start small -- they establish better and are less expensive. Seeing lots of nurseries going out of business on craigslist and selling trees for cheap cheap.

The arborvitae here in the Portland Metro Area are decimated by this winter's snow and ice. The Western Red Cedars are looking good. They do need a lot of water.

iforgotitsonevermind awesome post. Thanks. I need something at least 6 feet tall right now. I'm tired of seeing neighbors on their deck as much as I am.

Left out light factor.. leyland's will thin in shade. Thuja occidentalis & plicata's will take partial shade.

"Leylands are beautiful but don't root as fast as top growth, so they can topple over in snow / wind / just because"

The toppling over is due to fast but badly shaped root growth of potbound trees, not due to slow root growth. The coiled roots at the bottom of the pot end up like a hinge, and the tree just turns over when it gets large.

When planted properly, the roots grow OK, and support a huge tree perfectly well; this one is 36 metres tall:
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The problem of course is that trees like that aren't suited to most gardens, so people end up cutting the tops off leaving hideous monstrosities like this . . .
{{gwi:489057}}

Resin

Thank you Resin for clarifying that. Have a lot of varieties of leylands in my yard. One was planted before we bought the house and is growing very fast but we've had to chainlock it to the garage to keep it upright. The snow brought down a Silver Dust this winter but we straightened it back up and have it chainlocked to lodgepoles. The rest were tiny babies that we planted and were not root-bound and are not chainlocked so I have hopes they'll root correctly :-) Those little babies have started growing now. Glad these trees take to regular and judicious pruning. Pruned the big already-here one last winter a bit and it's fine.

Have 2 Naylor's Blue and 2 Emerald Isles which supposedly are smaller than the straights. Have a bunch of Castlewellans which I know get huge. Love those bright gold-green tips in spring.

So far I've been surprised at how well these trees have survived in my yard, which can be very wet in fall / winter / spring. A lot of other evergreens have croaked.

If a person is able to cultivate an evergreen that grows as fast and bushy as a leyland, and accepts pruning as well, and then stops at around 25', that person will become very wealthy.