when it comes to sound?

i've been playing a riff, and people tell me it sounds better on the 5th of the top string rather than the open A

i know there are the same exact pitch, but the tone is a little different.

can somebody please elaborate on that for me?
I think its easier to keep the same hand position when moving between power chords. when you use the open A, your hand changes position than if you were to play a normal power chord.
The thickest string is referred to as the bottom string.

It's much easier to put vibrato on the fretted note, and as the string is thicker it will have more bass in the sound.
I'm a violinist, first and foremost. On violin I always try to avoid open strings because it has a much harsher sound and you can't vibrato or anything. I've sort of just adapted that to guitar where I will fret the note rather then use an open string. So long as that's the practical thing to do. I've never thought about it, just done what I was used to. Maybe that helps?
It just gives a slightly different tone. In some cases, like if your playing some chicken pickin country licks, you want to use the open strings.
Thicker string gives a more dominant, bassy sound. It's a matter of preference and what's practical i guess.
All these guys have pretty much said what I was thinking. On a personal note, I play whatever feels best with the rest of the riff. If I'm playing open notes and/or around frets 1-2, I'll probably play an open A...etc. But when I'm playing bass, I try to play as much on the E string as possible, to keep the deeper bass.
Fender Hot Rod Deluxe
Gibson Faded V
Warmoth Strat copy
Epiphone Hummingbird (FS!!)
Ibanez SR400QM
Fender BXR100
Reggae Bass Covers mahn!!!

The fifth fret of the E string is more useful if your sliding up to it.
***Short Sig***
On songs like Queen's Dragon Attack it sounds better, but really it doesn't mater that much.
Fender Lite Ash --> TC Polytune --> Digitech Whammy V --> MXR Phase 90 --> EHX Small Clone --> Strymon Orbit --> TC Flashback X4 --> Rivera R55
How would you use a vibrato on the open string without using a trem?

You could do it behind the nut, doubt it would be very convenient though .