They are entirely interchangeable. Use whichever form you find most convenient to pronounce.
If you plan on writing poetry, this interchangeability can come in handy because these forms can adapt to whatever rhythmic structure you're aiming for. ;)
As Elena said, they are both interchangeable. However, the "-es" form sounds a bit antiquated if you use it in normal conversation. It is in fact more in use in written german, rather than in spoken german.
For a word where both -es and -s are allowed genitive forms, -es is indeed considered more poetic, but could also be interpreted as outdated as stated by Maico Heere.
The Duden says that -es is preferred in constellations where the genetive is prefixed (e.g. des Tages Hitze) or within compounds with interfix-s (des Geschäftsfreundes).
