Should I recommend accepting a manuscript with major revisions because of a strong topic, but quite poor content?
Jeff Offutt has a very insightful scheme for calculating the final recommendation based on technical, presentation, and omission grounds (source):



The fields in the matrix correspond to different types of problems:


  Technical Problems
  Minor: Mistakes in background, related work; 
  Moderate: Does not affect the key results;
  Major: Changes the key results;
  Critical: Negates the key results.
  
  Presentation Problems
  Minor: Typos, spelling, grammar; 
  Moderate: Make understanding the paper harder  (organization, notation, repeated grammar);
  Major: Prevent understanding of part of the paper;
  Critical: Prevent understanding or evaluating a key result;
  
  Problems of Omission
  Minor: Omitted background, related work;
  Moderate: Not part of the key results;
  Major: Missing in the key results (proof or experiment, 
  lack of control in experiment);
  Critical: Must be in the paper to evaluate the result 
  (experimental study, etc) or not enough results.


The source also provides additional information and context.

Based on this scheme, I think the appropriate recommendation in your case would be a major revision.