Will an abdominal MRI tell as much as a colonoscopy?

Colonoscopy better. If you are looking at the colon, colonoscopy is better.
No comparison. Like xrays, MRI is an indirect visualization modality. Unlike colonoscopy (a direct optical exam of the colon lining), small lesions may be below the detection threshold of MRI scanning. Also, colonoscopy affords the opporunity to biopsy the colon lining, to remove upon detection colon polyps (finding them on MRI will only prompt undergoing a colonoscopy to get them out), as well as therapeutics.
Depends. It really depends on the purpose of each test. A colonoscopy specifically looks at the lining of the colon and sometimes the terminal ileum (the last part of the small bowel). The MRI can evaluate many structures outside of the colon. It may also be used to stage rectal cancer to look at the tumor and associated lymph nodes. This is a special application of the mri.