Not much. Both terms were created from the underlying term "Situation", and describe a computing model whose primary driving force is the situation of the user.

Dictionary.com says:

sit⋅u⋅at⋅ed
adj.

1. Having a place or location; located: a cabin nicely situated on a quiet riverbank.
2. Supplied with money: a family that has always been well situated.

For "situational" it simply says that it is the adjective of "situation".

We prefer "situational" as it is not tied as much to a particular location as "situated" seems to be (see definition above). See also logical vs. physicial situations.

The page seems to be a little out of date, but there's a very good description of what they call "Situated Computing" here.

Some quotes:

Despite recent technological breakthroughs ..., many people still find difficult to interact with computers while maintaining their normal flow of work in the real world. The key reason for this dissatisfaction arises since applications have no knowledge about the user's real world situations and they are conventionally developed to provide explicit functionalities.

Situated computing is an activity-centered paradigm to enhance the user's interaction in a mobile environment through applications that can understand the user's context and activities... The main objective in situated computing is to enhance the human-computer interaction through simplified interfacing based on the user's real world situations...