Depending on the individual office, both are probably handled with equal care. Ross Perot had this down to an art. We got the point and sent a response as soon as we were able to. This involves a lot of paper handling and approvals from several people before it can be sent out. Believe it or not, sending 2 letters on separate issues will probably get you 2 responses quicker than a you could expect a combined response.
Every Congressional office is different.
The amount of mail we get boggles the mind. Anything that takes staff time reduces the probability and prompness of a reply. In reality, all staffs hate them and put them off until the next Congressional recess if they can, or even indefinately. Most U.S. Senate offices throw them directly in the trash, since their volume is exponentially more than that of a Congresional office. Just like postcards only worse.