well today i went to radioshack to replace my old blue led with a new one. now i got the exact same one, which is 300mcd and 30mA, but the other option was a 2700mcd 20mA led. what should i have gotten? the 2700mcd is probably much brighter(too bright for my needs), but more efficient.

One of them is rated at around 5volts, and the other at 3.7, IIRC. I'm glad you picked up the same one!

the 300mcd one is the better choice right?

If it matches the existing one, then I'd say yes. I had only noticed that they sold two different blue LEDs a short while ago myself.

VC, that "rating" you're looking at is probably just the voltage drop. All else being equal, a higher voltage drop gives you a higher efficiency if you use a current-limiting resistor, since the voltage the resistor sees is lower, thus the current is lower. But, all is not equal. One of those two is going to be more efficient at lower voltages, but which one? You'd have to get the datasheets or just buy them both an test them to find out.

As to the implied question of how many millicandelas is "enough", that's irrelevant, since you can always add a bigger limiting resistor to bring the brightness down if it's too bright. Since large limiting resistors are good for efficiency, an LED that's "too bright" might actually be a better buy than one that's just barely bright enough.

Interesting point, Tangent. I may check into that further... I don't believe that the 5v drop piece is brighter than the 3.whatever v drop piece, either. (not that you implied that, but many people might assume such).

On another note, they (RS) charce around 2.50-3.00 for a blue, and 4.99 for a white. I just got in an order from Mouser, and fairly bright clear blues were .69 each. I tried them last night, and they are almost as bright as one of the RS LEDs, and way brighter than the other RS LED.

Like most things at RS, buyer beware.