From a strict numerical standpoint, there's no difference: there are six sharps in F♯ major and six flats in G♭ major, so in this sense they are equivalent.

But there are some other musical factors you may consider:

• What is the instrumentation? Some instruments—like brass—are more comfortable in flat keys. Although players should be able to play in all keys, your brass players will make fewer mistakes in G♭ major, trust me :-)
• Similarly, what level are the players? High school brass ensembles could certainly play in G♭ major, but I don't know many high school directors that would feel comfortable giving their students a piece in F♯ major.
• If you have transposing instruments, you may find yourself choosing one key over the other to make the conductor's reading easier. A B♭ clarinet is written a whole step higher than it sounds, so for ease of reading, you may choose to write everything in G♭ (otherwise your clarinet is written in G♯ major!). True, you can have flats and sharps in the same score, but again: if you can make the conductor's job easier, you might as well do it.
• What does the piece do harmonically? If it often uses, e.g., ♭VI (or even modulates there), it'd be easier to write it in F♯ so as to prevent any E♭♭ chords.

And so on.

From a strict numerical standpoint, there's no difference: there are six sharps in F♯ major and six flats in G♭ major, so in this sense they are equivalent.

But there are some other musical factors you may consider:

• What is the instrumentation? Some instruments—like brass—are more comfortable in flat keys. Although players should be able to play in all keys, your brass players will make fewer mistakes in G♭ major, trust me :-)
• Similarly, what level are the players? High school brass ensembles could certainly play in G♭ major, but I don't know many high school directors that would feel comfortable giving their students a piece in F♯ major.
• If you have transposing instruments, you may find yourself choosing one key over the other to make the conductor's reading easier. A B♭ clarinet is written a major second higher than it sounds, so for ease of reading, you may choose to write everything in G♭ (otherwise your clarinet is written in G♯ major!). True, you can have flats and sharps in the same score, but again: if you can make the conductor's job easier, you might as well do it.
• What does the piece do harmonically? If it often uses, e.g., ♭VI (or even modulates there), it'd be easier to write it in F♯ so as to prevent any E♭♭ chords.

And so on.