I strongly encourage a transitional year
My department chair always encourages residents to do a year of prelim medicine, but I attribute that to him not having been a resident recently
The main skill of internship should be determining whether a patient looks toxic. The actual management is important as well, but the main issue is the "gut instinct" that a patient is sick and needs further management
The main benefit of doing a year with a lot of elective time are greater exposure to relevant fields (rheumatology, neurology, orthopedics, radiology, etc). It is not clear that doing more than 5 ward months really make someone better at internal medicine- the marginial learning of months 6-12 on the wards is far outweighed by a more broad clinical experience.
Plus, the real learning for most internal medicine programs is in years 2 and 3. Year 1 is mostly about workload, whereas transitional programs tend to have a lighter load with a more educational focus.
BTW, consider internal medicine programs with lots of elective time, instead of a transitional year. I did a year of internal medicine at Bassett Hospital in Cooperstown, NY, and it was an awesome year. I felt like I learned plenty of medicine in the 5 ward months and 1 ICU month, but also benefited from the 5 months of elective. And since it wasn't a transitional year, I didn't have to do any surgery