Your best option is to purchase in your own domestic currency.
Other exchanges may have a variation in the spread and liquidity but if you were to try and convert to a different currency to take advantage of this the regular banking system would make whatever profit was originally available from fees and conversion rates (most likely).
If there is a chance you own a multi-currency bank account, USD has the best liquidity between all the exchanges.
Once you have bitcoin, trading between sites and currency is known as arbitrage (and cross currency arbitrage) this can be automated or done manually, if you can work out the fees involved.
Remember trading between any currency can be risky and swings may happen between fiat pairs that can disrupt your trading.
Don't risk more than you can afford to loose if this is the route you decide to take.