Tilt-Shift creates a band of sharp focus with gradually increasing blur either side, while Iris Blur creates an ellipse of focus. Field Blur is the most flexible: isolated points are positioned freely on the image, each with its own blur amount, and the software interpolates between them. Intuitive controls appear directly on the image, and with quickly updating previews, designing complex blur effects is extremely intuitive.
The three blur types can be combined and previewed together before you commit to them, but this can get unwieldy. Applying two Iris Blurs, for instance, creates two sharp circles on the image. However, when combining a Tilt-Shift and Iris Blur, both effects work independently so blur values are compounded, potentially leaving nothing sharp.