Hello,
I have to choose between getting all the cards for one deck or the other.
The two decks are sniper and elvish overrun.
please help me decide

What is your elf decks list?
The decision ultimately lies with you. Elves may be more competitive but which deck will you enjoy playing more? The amount of money that you spend on one or the other could also affect your decision. The number of cards you have for either deck could also make a difference. But in the end the decision lies with you, who will be purchasing the cards.
I'll make a new thread for that deck.
If you think you would enjoy playing either deck then I would suggest the elvish over run. You can play it as a casual deck and it's a pretty easy deck to spend a little money on and turn into a tier 1 Deck. Mono-Green elves right now are still competitive although I haven't seen one hit a top 8 in a while. Green has sick growth especially if you use the awesome
Oran-Rief, the Vastwood and Garruk.