I am wondering what would be the best light source to use in my 90w low pressure sodium streetlight for lighting up my backyard. Below are listed some of the benefits and drawbacks of each light source that matter to me. I like "softer" CCTs, low glare, decent CRI, inexpensive replacement lamps, and good optical distribution.

• SOX (90w, which is what the luminaire was originally designed for) is incredibly efficient, but has no CRI, an unpleasing CCT, and costs the most to run because lamps cost a lot of $$$. It also seems like it would be a little bit too bright.
• PL-L (55w 2700K) provides for good CRI and energy efficiency, low glare, and similar optical distribution to SOX, but I am not sure how well it would perform in cold weather conditions. It also requires a ballast and socket change.
  It is also the cheapest to run because lamps can be found for very cheap, and you get many color temperature choices.
• LED SOX retrofits also provide for decent energy efficiency (the lowest amount of energy used), long life, and decent CRI, but I am not sure about the quality of the light produced due to the way the fixture's optical assembly was designed and most retrofits having bare LED chips that point directly downwards.

Bias against certain light sources aside; what do you think would be the best light source for my application, do you have any recommendations?

Personally, if it were my fixture, I'd run it as is.

These PL&S luminaires are, quite frankly, rarer than hen's teeth now, so if I were you I'd preserve the authenticity of the luminaire by running its proper lamp, or at least a lower-wattage lamp (with a lower-wattage ballast) instead of desecrating it, lol.

Just my 2c.

With LEDs there are many variants to chose from, from directional intended to be used at places where the lamp was (optically) bare, to halfplane (intended for rfractor optic based fixtures) to really omnidirectional models. But I would expect the usability would correlate with its price...
Otherwise it also depends how much you wish to modify the fixture...

 Honestly , I had several PL lamps in my yard and I HATED them ! They absolutely SUCK in the cold and personally , I wasn't a fan of the clarity .  I don't know about the 55w but the lower wattage will lose 1/2 their lumen output during temps under 55°F
 Now , that being said , 55w would be the closest size to the SOX . Plus , socket install wouldn't be too difficult and shouldn't require much if any mods to the fixture .  (If they don't line up , you may not have to put both screws in the lamp holder to avoid drilling new holes)

 SOX would be the perfect choice BUT replacement lamps are hard to find and rediculously priced ! . Lower wattage would be a nice idea but again  , finding the parts without spending a zillion dollars !

 LED SOX : Nice idea BUT , you wind up with something that doesn't utilize the optics of the fixture , provides much higher color temps , and is also rather expensive .

 Maybe a slightly higher color temp of PL (3,500k) might look better .

 In the end , I think PL-L might wind up being your best option .

my 2 cents would be LED-SOX lamp assuming you can find one "you like"
my reasoning - rare fixture left original - low cost to run and being a YARD and not a road you likely actually do activities under this light and having good CRI is important and depending on you install and neighbours you can control light trespass by using a directional lamp to reduce sideways light - OR use a 180 - OR omni directional if it is mounted lower / no neighbours within sight of lamp
if the temps drop below -5 (45F??) FLORO's get dim - I have a NON heated front room that HAD a 26 watt self ballasted CFL and it failed to strike at +10F and now has a 22 watt circline on a mag ballast and it is near as zero F right now and it runs but is so dim it is nearly NOT worth turning on 

PL-L would only really be a problem in the winter. The rest of the year is usually pretty warm.

 I have seen the SOX retrofits in 2200k & 590nm  . The problem is that 95% of them are just a flat panel with a heatsink which means that unless you're gonna mount it 30' in the air you're just gonna get a small patch of light on the ground .

That’s why I think PL-L would be the best alternative. If we had the 360 degree SOX LED retrofits, they would be a better contender.

PL has been fine in the -10 deg C winters here. Though for the past 15 years it has rarely been below -5. We ran them for about 20 years until the luminaire simply rusted away together with the ballast.

The new outdoor light has a standard e27 socket so there's a LED bulb in it now, has been going strong for the past 5 years or so. But locally i haven't seen any SOX led replacements so i can't give an opinion about those.

Keep the future availability of mercury containing lamps into account if you go for PL. No reason not to use them, but it is a reason to stock up.

Would it be possible to fit a bog standard fluorescent in there? Those work decently well in the cold once warmed up. In an enclosed luminaire they'll retain some heat. Half the street lighting here is still standard T5 fluorescent stuff.

Finally, at DBL lighting you can get the (freshly manufactured, potentially Indian) SOX 18 till SOX 55 lamps for 30 euro. Perhaps you can put in the smaller SOX 55 and the appropriate gear. I'm sure someone would like to trade you a 55w ballast for your 90w one.