I read the Steam description of the difference in their knowledge base, but it didn't actually clarify anything for me. In both cases you end up with a code that one can use to add money to one's Steam wallet, correct?
I have a slight preference for a wallet code instead of a gift card because I noticed that the sites that are selling wallet codes online seem to have a $30 denomination, which is what I prefer. Gift cards only come in $20 and $50. I've noticed that the sites selling wallet codes seem to all be overseas for some reason, which makes me a little bit leery as to their legitimacy.