I go w/email; it's far easier, far faster, and uses less of the lawyers' time as well (as opposed to taking 45 seconds to open and read your letter, they take 5-10 seconds looking at it online). The formal letter looks more impressive, so if it's a place that you know won't make the decision immediately, and you really just want to knock their socks off (if you got a callback, it's unlikely you'll need to do that anyway), you can do a snail mail letter.