Neither will be a sound barrier. Bamboo is popular because it is cheaper than fencing, but it still requires an investment both for installation, including a root barrier and subsequent upkeep. Needs consistent irrigation to remain fully attractive on most sites here in our dry summer climate, plus is apt to become infested with mites. Also sheds rather abundant litter in summer, as do many broadleaf evergreens.

In most situations the optimum is probably a fence with a mixed planting of varying heights in front, rather than a uniform hedge. Rather tall plants can be worked into a narrow sideyard if you do not insist on being able to travel a straight path with all the planting plastered against the property line. A delightful, meandering woodland walk effect can be created instead with informal planting, I have seen a small property with the side yard planting so as to look quite a hike in the woods.

I am working on a screening row that is a random series of arborvitae, Leyland cypress, and western hemlock. They grow at different rates and have different color and density foliage, so it doesn't look like the usual regimented evergreen fence. Staggering them a bit, rather than planting them in a straight row helps it look less regimented, too. The Leyland cypress were about 5 feet tall when I planted them, and in three years are already between 15 and 20 feet tall.

Along the back yard, I am working on a line of camellia, Portugal laurel, photinia, and euonymous in groups of three or four, again, so it doesn't look so boring.

Fun monkey puzzle tree in the pic. I love them (especially in someone else's yard).

I'd vote for the arborvitae, though I agree with the others they're boring boring boring as a line (I have one line of them that was already here and I like the visual privacy).

I'd do the arborvitae and then diversify it somehow with some plantings in front of it (though not too close).

Wayside has what they call the fastest growing arborvitae available, though at $30.00 bucks each I wouldn't call them an inexpensive solution compared to larger ones I've seen all over town for $9.99 of 'Emerald Green'.

Piano's idea is terrific--I bet it looks much more interesting than a solid row of the same thing. I'm planning on something similar in my front garden, but my neighbor's deck is on their THIRD floor in back so I really like the privacy the arborvitae provide.

Piano, any pics of your plantings? They sound terrific. I'm definitely entertaining a similar notion along my front garden, and something to make my arborvitae brigade not looks so regimented.

Let us know what you decide to do, and how it works out.

Take care,
Grant

Here is a link that might be useful: Steeplechase Thuja from Wayside

I've seen a stunning "fence" of timber bamboo, all growing in a straight line. I don't know how it was achieved, maybe with just a good barrier. I have P. nuda planted without a barrier and no supplemental water and it always looks great, but I have lots of room for it to spread. I also planted a couple of 2 yo timber bamboos which also never get watered. One put up a lot of new canes this year and the other didn't. Maybe it needs more water.

I would suggest a series of trellises as tall as you need, and grow climbing Tea roses, Zepherine Drouhin (thornless, continual bloom for me), etc mixed with Clematis for a lot of bloom and color. They would be narrow.

Ron's woodland stroll idea sounds great. I've seen yards like this and they are very low-maintenance and appealing with winding gravel paths and assorted evergreens, throw in a weeping Japanese maple or two.

Here is a photo of my bamboo clump. It is very useful for poles. I am trying them for tying up tomatoes this year and also for deer barriers. This one is not long and narrow but perhaps with a barrier it would grow that way, though a timber bamboo would be so much more impressive.

Here is a link that might be useful:

NICE clump of bamboo. It looks very, very happy. I'm envious that you've got a constant source of stakes and poles. Do you usually need to let them dry/harden?

In any case, thanks for sharing the pic.
Take care,
Grant

Grant- thanks, the bamboo always looks nice, even in winter. This year when I started cutting poles I went for the biggest bases I could see. When I started pulling them out I found that they were this year's sprouts, and hadn't even finished growing at the top. They were very weak and bendy. Now I have a program of cutting through the center of the patch to open it up and will continue to the other side, then I will cut more from the inside of the clump. I'm cutting out small and medium canes which I decided were older and leaving the really large ones to get stronger. I don't know if they need to age after cutting to get stronger? They do fine for tomato stakes and deer barriers but I'm looking forward to the timber bamboo for more length and strength. That may take a few years. Any ideas how to get timber bamboo to grow closely single file? I could ask on the bamboo forum.

Leyland Cypress - single - can become 45' wide in 20 years. Oh the suprise that awaits many homeowners.

Check out this link of mine below about that tree.

(I'd go with columnar hornbeam for a narrow upper screen, with arborvitae or columnar forms of yew for the lower screen)

Here is a link that might be useful: Does Money Grow On Trees? - Leyland topic

Columnar ('Fastigiata') hornbeams grow fat also, in fact quite similar to Leyland in this respect--in those districts where the latter grows broad (other places it does have a columnar habit).

'Frans Fontaine' hornbeam is supposed to remain comparatively narrow, but is also comparatively new and uncommon here--not many of much size to look at and assess.