Bicycle Mechanics - BB spindle length

I'm getting ready to buy a replacement for a worn out shimano lp26 68x116 bottom bracket. It looks like 116 isn't an option for spindle length on the newer shimano bb's (i'm planning on getting a un54), but they have 115 and 118. Sheldon's bb size database lists the usable lengths for my crank at 116-121, so I'm going to order the 118 length.

I know it's only 1 or 2 mm difference, so I guess this question is mainly for curiosity, but what (if any) effects would changing the spindle length have? Does it just mean having to adjust the front derailleur? Does a shorter spindle length give you a 'better' chainline?

For all practical purposes 115 = 116. Unless your crank arms or the teeth of the rings are a hair's breadth away from contacting the chain stay neither you nor the chain will notice the 0.5mm per side.

The chainline change going to 118 would depend on which gears you use most consistently. If the present chain line is angled inward as it runs to the rear then going another 2mm out will very slightly increase that angle and technically that is bad. Unless you enjoy a combo where the run is getting to a pretty sizable angle then 2 mm again won't mean diddly. But if you like running the big up front and middle 3 in the rear for the most part then you may as well go for the 115 provided the .5 mm won't mean an issue with the crank tips or ring teeth at the chain stay.

Technically you're also slightly affecting the Q value for your foot placement. For the sake of a 3 mm spread I doubt it would make a difference one way or the other unless you are a serious racer or have some sort of knee condition which would benefit from any tiny degree of help. But otherwise the average Joe or Jane would never notice in the slightest a 1mm shift one way or 2mm the other from where you have your pedals now. Technically going narrower is supposed to improve the Q for most folks. So it's another check mark for the 115 but I sure wouldn't bother if the chainline would see an improvement by going longer instead of more narrow.

Excellent! Thanks for the information and advice, that shed some light on what determines a good chainline and I hadn't even heard of q value before.