in all honesty, it all depends on the character and the creator. if they live in equestria or something, then it would be better to make up your own land for them to be in, or an alternate universe. these creatures are supposed to be god like, and having them all roam around in something like ponyville really takes that away.
as for the creator, it would be so so much better if you just chose one instead of going on both.
trust me, everyone wants to do one or another, i had that problem a lot when i was working on one of my characters.
i wanted her to be a unicorn, but i also wanted her to be a pegasus. (at the time, at least.)
a simpler way of cleaning this up was to make them an alicorn, right? well it depends on if the character is a sona or not.
but anyways, does your pony like to make crafting and all that good stuff? make it a unicorn
does it like to take a stroll in the big blue sky? mess with the weather and all that? make it a pegasus
does it like basic simple life? none of those fancy ol' powers, and to simply roam the land with sweat on their faces? then make it an earth pony.
this doesnt have to be the end all be all of stuff, this was just an example of what i see in the show.
you can have a unicorn who farms for all you care!
just dont choose a alicorn if you dont know what you want. really think about the character first, then the race.