SPF and SenderID are similar to each other in the way they act, but the differences between them are in what they are designed to target (at least how I see it).
Both OpenSPF and Wikipedia say that SenderID and SPF are designed to address different problems. SPF is designed to work with SMTP while SenderID is designed to protect the content of a message. I think it’s a stretch to claim that it’s like comparing apples to oranges; let me explain myself.
SenderID can be implemented to work just like SPF, but also introduces (and recommends) features to protect the content of the message. It does this by examining the message headers and extracting the Purported Responsible Address (PRA). None of these message headers are required by the SMTP protocol, they are extra headers that can be found in RFC 2822. Anyways, these message headers are basically part of the message content and are used to display information to the end user. The From: address is the one that the end-user sees and is part of the content of the message. Because most users will assume that the From: address is the one that sent the message (especially for phishing), SenderID can be used to protect against forged email addresses that are displayed to the end-user (the one that the sender wants the receiver to think the message came from). In other words, it protects against forged content.
SPF, on the other hand, can also work against forged Return-Paths (also known as envelope senders). It’s basically a connection-level protocol that protects against spoofing the envelope sender for a domain. While this can work in the case that a spammer spoofs a bank (phishing), SPF is not explicitly protecting the content because it can do all of its checking before it even receives the message content.
Both protocols protect against forged senders, but one does the work at the connection level while one does it at the content level. SPF protects against bounced messages and SenderID does not, but that doesn’t mean that SenderID couldn’t be implemented to do the same thing. There are, of course, some parts that do not carry over between SenderID and SPF but to say that comparing the two is comparing two completely different things is inaccurate. I think that there is too much overlap.