A content upgrade is related to specific blog content you have posted. (You could even tie it to social media content you share.) The customer is already consuming that content, so you know they're interested in the topic. It's the perfect time to offer extra "bonus" content in exchange for their email address. They get valuable information about a topic they're interested in and you get to grow your list. It's a win-win!

You can use both! Content upgrades are great because you're offering something extra, connected to content they've already shown an interest in. You're giving them extra value when they're in a receptive state. The content upgrade can also be useful for segmenting users based on interests. Not everyone is going to read your blog posts though, so traditional opt-ins are still important.