What I wan't to be depends stongly on how the conversion process works. Specificaly, how literally the mind is translated and how cutie marks/special talents transfers.
I'm assuming that the races are actually balanced - so what are the advantages of earth ponies? Well, applejack has superhuman contentiousness and downright supernatural skill at a wide veriity of physical tasks. Pinkie can break the fourth wall, and have shown crazy engineering skill. Most of the nobles shown at the gala wee earth ponies.
Seems to me that whatever earth ponies have, it's mental. That is WAY more usefull than any physical trick like flight or telekinessis. But it also means that advantage might not actually be given to someone who transforms into one rather than being born like that.
Meanwhile, the unicorn magic we've seen so far... Well, there are lots of powerful and useful things, but they wouldn't be usefull for any of the things I do seriusly. On the other hand, they are said to adapt to the special talents... And things relating to things I'd conider special tallents of mine would be incredibly usefll, in my hooves even more so precisely because I'm tallented at them. What magic spells would a cognitive science cuti mark, or an artificial intelligence cutie mark, or a decition theory cutie mark, or even a web surfing cutie mark, give? Presumably ones that let you manipulate brains, program and/or train artificial intelligences, help you make better decitions, or download information rapidly into your brain. ANY of those and I'd eventually take over the world.
So; do I get superhuman mental attributes from choosing earth pony? Do I get spells related to MY special talent(s) if I chose unicorn? My answer depends a lot on both of those.