I've found that you're much more likely to get a response through snail mail than through email. On the other hand, I find that when you query by email, you're much more likely to receive a response from the agent (rather than an assistant) and receive it faster. So there are advantages/disadvantages to both. Having said that, next time I do a round of queries, I'm giving preference to agents/publishers who accept email queries. The expense does add up, especially when they want the first three chapters. I'm not in the US, but all my queries go to the US, so it's not cheap to send a package containing three chapters. Since they all claim they can make a decision after reading a page or two, I think they should only ask for the first 2-3 pages (as some agents do) if they want a writing sample with the query.
There are a handful of agents I'm interested in querying that specifically say they don't want email queries and I'll follow their guidelines. Everyone else is getting email.
There are a handful of agents I'm interested in querying that specifically say they don't want email queries and I'll follow their guidelines. Everyone else is getting email.