There's no difference between "여기요" and "저기요".
Korean usually doesn't put any meaning into these words when you get a waiter's attention. Which means, you could get attention in various way by saying like as below.
"주문이요", "이모(when you call the waitress that seems older than you)", "누나 or 언니"(when you call the waitress that seems younger than you), "아저씨"(when you call the waiter), "웨이터" (when you went to fancy restaurant), etc.
But still Korean uses 저기요 (or 여기요) generally.