No experience...however, Dulcis, hardy to about -5F, and Decora, hardy to about -11F, will very likely die unless you go to extreme lengths to try to protect them with tarps and mulch. Just my 2 cents...

Both hold up fairly well to 0F with light leaf burn, both start to burn hard below that, the Dulcis will hang in there with significant leaf burn to maybe -3F or so depends upon wind, duration etc but will defoliate much below that and culm damage as well once you get below -3F or so. The Decora will hang around a little bit longer, both had serious damage on 6 year old groves at -8F with strong winds but both held the lower 25% or so of the canes and releafed, both had lots of totally dead culms as well, the Decora did releaf higher up. Similar in hardiness I'd say.

The Dulcis will produce fewer, but fatter culms, I'm seeing a tad over 1.5 inch diameter at around 16-17 feet in height, Decora will produce lots more shoots but thinner and taller canes, around an inch plus by 18-20 feet tall.

inversa-
Is your experience in temperature conditions as low as the OP's. I wasn't clear if you were implying they could survive that over the long term.

The OP is in z6a, as I am - are you reading their 'below 20F' as 'below -20F'? I think they mean temps may fall below +20F for 3 months which is similar to me - perhaps I read it wrong.

Temp ratings are deceptive at best, if temps STAYED below +20F continually for 3 months, then I'd throw out the temp ratings and expect lots of damage.

My post lists damages caused by periodic arctic blasts that can happen each year at those temps I listed which generally cause the damage as I described. Such is why I avoid temp ratings on my website, they do not tell the whole story.

kudzu and inversa, thanks for the input.

Here's a better explanation of my winter temps:
avg. min. temp. = -5F
avg. #days below 0F = 4 (at least 1 per winter)
Temp's hover between 20F and 32F for most of December. January and early February see temperatures rarely above 25F. During this time period, we usually have up to a week at a time where temperatures STAY in the teens to single digits fahrenheit with a day or two of 0F or below. During the end of february and early march, temps hover again in the 20 to 32F range. So, it would be more accurate to say roughly 1.5 to 2 months of temps that consistently STAY below 25F with periods consisting of consecutive days with temps below 15 to 20F.

Like you said, temperature ratings aren't everything. I'm in zone 6a based on avg. winter minimum. However, it gets cold and stays cold for a long time here. Cold hardiness of plants should not be based on the lowest temperature they can survive brief exposure to but a temperature that they can survive prolonged exposure to. I look at a plant rated to zone 6 and think "if it stays between -5F and +5F (ignoring wind/exposure effects) for a month straight, that plant will be in perfect health come spring". Very misleading.

If you have no bamboo at all, I'd suggest that you first try Yellow Groove, Spectabilis, or Bissetii. All 3 are at the top of the hardiness list and would all do better than the 2 you asked about. You'll read that Nuda is the hardiest but it hates wind and mine top killed the first 4 year while the above did not. Nuda is also slower to size up.

If you already have these and are adding either the Dulcis or Decora, the Decora will likely hold up a tad better in your winters - but not as good as the above. Your long stretch of below freezing high temps will likely result in cane damage most years and hold mature sizes down - wind is the real enemy and frozen ground plus wind you'll get damage similar to serious subzero.

inversa-
I was looking more at what the temps typically were for that Zone, where the lows are just above the hardiness ratings. While I agree that hardiness temps should be taken with a grain of salt, I have found that they do serve as a rough indicator. Further, I tend to be conservative in my advice to new bamboo gardeners: my experience is that you can have three or four winters with a plant where it survives just fine...only to get nailed the next season.
You have more experience with that Zone than I do, and I think your advice is good, so I am going to defer to your suggestions. Good luck, treeguy...you should be able to get something to work for you if you choose wisely.

Hey, thanks for the further input.

I currently have:
P. rubromarginata
P. atrovaginata
P. aureosulcata
P. aureosulcata 'spectabilis'
P. nuda
P. nigra 'henon'

So far P. nigra 'henon' has gotten the tallest, P. rubro. sends up the most canes each year, and the others have the best cane survival percentage. Mine are growing in a very exposed location, so I get a lot of top die back on all of them. You can check out this thread for more detailed info about my experience with these species:

I just wanted to add either P. decora or dulcis to my growing collection and was curious as to which would do better for me. There's a 20-25' grove down the road from me, so I know they'll survive long term. I'd be happy with 15 to 20 foot canes on any of my 'boos someday . . .

Compared to what you already have, I'd say Dulcis is about as hardy as the Rubro which I suspect burns pretty bad for you? Decora slightly better than Rubro.

That is based on what I see here anyway, it is always different in different places.

Yeah, the rubro does burn pretty bad for me. It consistently has 75% top kill while the nuda and atro may only have 33-50% top kill in an average winter. This winter was kind, everything completely defoliated, but I had 0% top kill on my atro. and nuda, 25% on my aureosulcata/spectabilis, and 50% on my nigra 'henon (rubro still @ 75%). Despite nearly complete top kill most winters, the rubro still produces the most canes. If they'd only get taller than 7' I could use the dead canes as posts/fishing poles/etc. . . .

I like the idea of fatter canes at the expense of a fwe feet of height. However, I probably won't see decent height on dulcis because of die-back, making me think I should go with decora. Thanks for the input!