By Paul Taylor BRUSSELS (Reuters) - After five months in power, Greek Prime Minister Alexis Tsipras has 48 hours to make a decision that will shape his country's future for decades, yet his negotiating partners cannot be sure which way he will jump. The 40-year-old leftist, elected in January on a promise to end austerity, must decide whether to swallow an unpalatable deal with international creditors to avert a default next week at the risk of losing part of his Syriza party, or refuse and risk economic chaos and ejection from the euro zone. European Union leaders made crystal clear to Tsipras at a summit on Thursday evening that their patience is exhausted and Greece will be given a "take it or leave it" choice at a finance ministers' meeting on Saturday.