They're both very nice trees and there's a pretty good chance that either will perform well for you. I've got a few of each growing here and all of them have done well. I'm in a full zone colder than you, so hardiness shouldn't be a concern.

If forced to choose between the two simply based on which one is the most tolerant and gives me the best chance of success, then I'd go with the spruce. While I've seen extremely attractive mature White Firs, full and robust, they're less tolerant of shade than the Serbian Spruce, which could be what you're referencing. White Fir is also more susceptible to winter burn on exposed sites than Serbian Spruce, but this has not been a major problem for me.

If you've got moisture retentive soil with adequate drainage in full sun, I recommend selecting the one you find most appealing.

My experience with A. concolor in Ontario suggests that they grow and mature well but are sensitive to extreme drought and will thin and/or loose branches. The year after year cumulative effect on a mature tree can be unsightly. With good siting on moisture retentive (but not wet) soils, you should have a lovely tree. Winter burn in most of Ontario shouldn't be an issue (I know of a mature one that has done just fine in Edmonton other than drought damage from a few years ago - zone 3).

No problems with Serbian spruce though it is reportedly less hardy but your not in zone 3 so no issue.

Thanks folks... @ Beng12 we get frost as lat as May 30 (but that would be a late one...early June is not out of the question either, though) Typically, our last frost is around May 20th or so, give or take.. not sure of that is a "late frost"?

@maackia, the site is full sun... winter burn I can see being a problem.. There are 5 concolor (orignal species) planted at my workplace. This year 2 got burned... but they are exposed to NW winds...open space and we got hit hard for 4 weeks this years with -25 temps and cold NW winds... Doesn't adequate moisture before freeze up help mitigate? plus burlap in its early life... hopefully...

@Smivies, I would try to make sure it is adequately watered during long dry spells... true droughts are rare in Ontario, wouldn't you say? We certainly haven't had a drought in years...

thanks to all!

My understanding is that the bruns is quite narrow (under 15' even after a few decades) but the blue cloak can and will exceed 20' - the wider girth of the abies might swing my decision to go with it instead. I want to locate it in a spot that will also screen out some neighbors over time... might put the omorika in the backyard where I actually need a narrower screen... can I safely assume these are accurate width expectations?

thanks!

Don’t know where you are in Ontario but Kingston went 8 weeks starting in mid-June without rain in 2016. That was extreme but stretches of 4 weeks are not that uncommon.

By late frost I mean later than other, nearby elevated sites. I can say that newly emerging sprouts on concolor fir (or any fir I have) cannot tolerate freezes, but spruces have some tolerance.

Smivies, yes we've had those kinds of stretches in the GTA and even longer.. just didn't think that was severe enough to kill trees (assuming you are watering anything that was planted within 2 years.). I an pretty diligent about watering anything I plant regularly until well established. I'd figure a 5+year fir would be fine during dry spells. Don't they originate in the drier climate of western north america?

Abies isn't a dry climate western genus. A. concolor falls somewhere between Western Hemlock and Ponderosa Pine for tolerance to drought.

Didn't know that! Thank you for the info... I figure during those dry spells, i'll get out the soaker hose and keep things moist... would you say that even bigger mature trees are susceptible?

Bruns and Blue Cloak are both slow growers. How quick do you want to screen?

Both are beautiful by the way.

Blue Cloak and the other very glaucous cultivars don't winter burn as much. The species depending on seed source burns quite easily in very cold windy winters but recovers just fine.

I am pateint! I can wait... if in 10 years I have 7' trees I'll be happy

Bought both today... the Blue Cloak is a teeny, tiny thing! 1 gal and maybe 6" or 7" tall! Not sure if I'll plant it yet as I'm afraid I'll not see it in the lawn and mow over it! That is the only size they had... the Bruns is a 5 gal, 5' to 6' tree... a little gangly and awkward but was assured that it will fill in and straighten out. Now I have to decide exactly where to locate them... Also picked up a tulepo tower black gum and a gardenii fothergilla... so some nice and unique specimens to liven up my landscape...