Infusionsoft and ConvertKit have little in common, except for the fact that they are both email-focused marketing automation tools. Everything from their list of features to target market are on the opposite ends of the spectrum.
Infusionsoft is easily the much better software. Its "business-in-a-box" approach means that customers don't need to buy any other marketing software; Infusionsoft will handle everything from acquiring leads to accepting payments from them.
While this might be perfect for a small business with a growing sales team or a seasoned infopreneur, most young startups, bloggers and very small businesses will be a little overwhelmed. It doesn't help that using Infusionsoft requires a steep upfront investment as well as an annual contract.
Generally speaking, if you don't have at least $100,000 in online sales, or more than 2-3 people on your sales/marketing teams, Infusionsoft will be overkill.
For such users, ConvertKit is a good alternative. The limited feature set makes it one of the easiest marketing tools to use on the market. The near-obsessive focus on email simplicity also means that it is arguably the best email automation software around.
ConvertKit's entire focus is largely on bloggers, infopreneurs and other heavy text-email users. This is visible in everything from feature selection to documentation quality.
ConvertKit's much, much smaller size also means you get more personalized attention. The ultra-low price and zero initial costs also makes it perfect for its target audience of bloggers, authors and infopreneurs.