Not quite sure how to put it, but they are different kind of "good passers". Shelvey might have better long range passing (diagonal balls). Personally I prefer Merino's kind of passing, as he releases it much quicker, recycles possession, and doesn't need as much space before he releases it. Merino is better when it comes to making us tick I think. Shelvey tries the Hollywood pass a little bit too often.
Like I said, he's more Cabaye than Shelvey. He's a good, confident passer but he's not going to be pinging constant worldies.