These 2 similar Shark stick steamers use water to clean and sanitize sealed hard floor surfaces.
According to product info and manuals, the difference between Shark S3101 and S3251 are as follows.
1, Swivel Steering
The S3251 features Swivel Steering for easy maneuverability whereas the head on the S3101 does not swivel.
2, Telescopic Handle
The S3251 has a telescopic handle whereas the handle on the S3101 is not telescopic.
3, Cord Length
The cord on the S3251 is 25 ft. versus 20 ft. on the S3101.
Apart from the above points, they seem to be the same.