Replacing my bottom bracket. I need help IDing a replacement So I have owned a Swhinn Network 3.0 (21-speed. Square tapered BB. All aluminum) for almost 5 months, and at about month 3 the bottom bracket started giving me some noise. I work in a bike shop on my UNI's campus so I opened it, purged, and re-lubed it. All was well. The it made noise again in two weeks. Rinse and repeat. Then again a few days ago it stopped turning. I got it to move again, but learned the bearing case (the little metal ring with the hooks that holds the little balls) had been flattened, pressed, warped, and demolished, so I removed both of them and put two more bearings in each side (22 bearings total) to occupy the excessively empty space and it worked fine for another two days until it started clicking again.

I am giving up on it and want to buy a non-servicing BB, but with all the acronyms and numbers and measurements I am not completely sure I can accurately pick the right one and not waste my (limited) money. Sheldon Brown is extremely confusing, and I can't make heads or tails of what he is saying, and can't seem to find my current BB on there anyway. What I have found that seem like they'll work and why/why not are listed below:

Shimano BB-UN26 - My bikes BB case (Terminology?) is 68mm, and the spindle is 125mm. It seems like the 68x123 might work, and thats only bringing each crank 1mm closer. Or I cuold play it safer with the 68x127.5 and tolerate the extra 1.25mm on each side (easy peasy, right?). My issue with it is the plastic cup. Is that a bad idea? Also, whould chainline matter at all if I get a BB with equal spindle length?

Shimano BB-UN55 (can't link, but it is on amazon) - Basically the same as UN26, but with an alloy cup instead of plastic. My frame is aluminum, so would this be bad to get, or good?

Are there any BBs you prefer that you'd recommend, or can I just run with one of these? I only have a week before I lose access to my tools, so time is also a factor, and if I can get it on Amazon Prime that would be most preferable.
You have done your homework correctly. You need a square drive bottom bracket with a shaft length as close to 125mm as you can get to fit a 68mm frame. I have taken out quite a few of the plastic cup units with no issues, but I prefer the metal one. But that is just personal choice. Take a look at the area of the chain stay (the tube from the rear axle to the bottom bracket) for crank clearance. Some frames run the big ring very close and bringing the crank closer may cause an issue. You can use a shim behind the crank- side cup to move it out to the same location as original. It is doubtful you will notice the difference if the nondrive side is a mm closer to the centerline. you will likely have to fine adjust the front derailleur when your done.